Chapter 6: Resistance Exercise for Impaired Muscle Performance

Muscle performance refers to the capacity of a muscle to do work (force × distance).¹¹ Despite the simplicity of the definition, muscle performance is a complex component of functional movement and is influenced by all of the body systems. Factors that affect muscle performance include the morphological qualities of muscle; neurological, biochemical, and biomechanical influences; and metabolic, cardiovascular, respiratory, cognitive, and emotional function. For a person to anticipate, respond to, and control the forces applied to the body and carry out the physical demands of everyday life in a safe and efficient manner, the body's muscles must be able to produce, sustain, and regulate muscle tension to meet these demands.

The key elements of muscle performance are strength, power, and endurance.¹¹ If any one or more of these areas of muscle performance is impaired, activity limitations (functional limitations) and participation restriction (disability) or increased risk of dysfunction may ensue. Many factors, such as injury, disease, immobilization, disuse, and inactivity, may result in impaired muscle performance, leading to weakness and muscle atrophy. When deficits in muscle performance place a person at risk for injury or hinder function, the use of resistance exercise is an appropriate therapeutic intervention to improve the integrated use of strength, power, and muscular endurance during functional movements, to reduce the risk of injury or re-injury, and to enhance physical performance.

Resistance exercise is any form of active exercise in which dynamic or static muscle contraction is resisted by an outside force applied manually or mechanically.^103,248 Resistance exercise, also referred to as resistance training,^8,9,167 is an essential element of rehabilitation programs for persons with impaired function and an integral component of conditioning programs for those who wish to promote or maintain health and physical well-being, potentially enhance performance of motor skills, and reduce the risk of injury and disease.^8,9,242

A comprehensive examination and evaluation of a patient or client are the foundations on which a therapist determines whether a program of resistance exercise is warranted and can improve a person's current level of function or prevent potential dysfunction. Many factors influence how appropriate, effective, or safe resistance training is and how the exercises are designed, implemented, and progressed. Factors, such as the underlying pathology; the extent and severity of muscle performance impairments; the presence of other deficits; the stage of tissue healing after injury or surgery; and a patient's or client's age, overall level of fitness, and ability to cooperate and learn, all must be considered. Once a program of resistance exercise is developed and prescribed to meet specific functional goals and outcomes, direct intervention by a therapist initially to implement the exercise program or to begin to teach and supervise the prescribed exercises for a smooth transition to an independent, home-based program is imperative.

This chapter provides a foundation of information on resistance exercise, identifies the determinants of resistance training programs, summarizes the principles and guidelines for application of manual and mechanical resistance exercise, and explores a variety of regimens for resistance training. It also addresses the scientific evidence, when available, of the relationship between improvements in muscle performance and enhanced functional abilities. The specific techniques described and illustrated in this chapter focus on manual resistance exercise for the extremities, primarily used during the early phase of rehabilitation. Additional exercises performed independently by a patient or client using resistance equipment are described and illustrated in Chapters 17 through 23. The use of resistance exercise for spinal conditions is presented in Chapter 16.

The three elements of muscle performance¹¹—strength, power, and endurance—can be enhanced by some form of resistance exercise. To what extent each of these elements is altered by exercise depends on how the principles of resistance training are applied and how factors such as the intensity, frequency, and duration of exercise are manipulated. Because the physical demands of work, recreation, and everyday living usually involve all three aspects of muscle performance, most resistance training programs seek to achieve a balance of strength, power, and muscular endurance to suit an individual's needs and goals. In addition to having a positive impact on muscle performance, resistance training can produce many other benefits.^8,9,10 These potential benefits are listed in Box 6.1. After a brief description of the three elements of muscle performance, guiding principles of exercise prescription and training are discussed in this section.

Strength, Power, and Endurance

Strength

Muscle strength is a broad term that refers to the ability of contractile tissue to produce tension and a resultant force based on the demands placed on the muscle.^182,192,210 More specifically, muscle strength is the greatest measurable force that can be exerted by a muscle or muscle group to overcome resistance during a single maximum effort.¹¹ Functional strength relates to the ability of the neuromuscular system to produce, reduce, or control forces, contemplated or imposed, during functional activities, in a smooth, coordinated manner.^42,219 Insufficient muscular strength can contribute to major functional losses of even the most basic activities of daily living.

Strength training. The development of muscle strength is an integral component of most rehabilitation or conditioning programs for individuals of all ages and all ability levels.^{8,10,90,168,221} Strength training (strengthening exercise) is defined as a systematic procedure of a muscle or muscle group lifting, lowering, or controlling heavy loads (resistance) for a relatively low number of repetitions or over a short period of time.^9,31,103 The most common adaptation to heavy resistance exercise is an increase in the maximum force-producing capacity of muscle—that is, an increase in muscle strength, primarily as the result of neural adaptations and an increase in muscle fiber size.^9,10,192

Power

Muscle power, another aspect of muscle performance, is related to the strength and speed of movement and is defined as the work (force × distance) produced by a muscle per unit of time (force × distance/time).^{11,182,192,210} In other words, it is the rate of performing work. The rate at which a muscle contracts and produces a resultant force and the relationship of force and velocity are factors that affect muscle power.^31,210 Because work can be produced over a very brief or an extended period of time, power can be expressed by either a single burst of high-intensity activity (such as lifting a heavy piece of luggage onto an overhead rack or performing a high jump) or by repeated bursts of less intense muscle activity (such as climbing a flight of stairs). The terms anaerobic power and aerobic power, respectively, are sometimes used to differentiate these two aspects of power.¹⁹²

Power training. Many motor skills in our lives are composed of movements that are explosive and involve both strength and speed. Therefore, re-establishing muscle power may be an important priority in a rehabilitation program. Muscle strength is a necessary foundation for developing muscle power. Power can be enhanced by either increasing the work a muscle must perform during a specified period of time or reducing the amount of time required to produce a given force. The greater the intensity of the exercise and the shorter the time period taken to generate force, the greater is the muscle power. For power training regimens, such as plyometric training or stretch-shortening drills, the speed of movement is the variable that is most often manipulated²⁹³ (see Chapter 23).

Endurance

Endurance is a broad term that refers to the ability to perform low-intensity, repetitive, or sustained activities over a prolonged period of time. Cardiopulmonary endurance (total body endurance) is associated with repetitive, dynamic motor activities, such as walking, cycling, swimming, or upper extremity ergometry, which involve use of the large muscles of the body.^8,9 This aspect of endurance is explored in Chapter 7.

Muscle endurance (sometimes referred to as local endurance) is the ability of a muscle to contract repeatedly against a load (resistance), generate and sustain tension, and resist fatigue over an extended period of time.^8,9,11,234 The term aerobic power sometimes is used interchangeably with muscle endurance. Maintenance of balance and proper alignment of the body segments requires sustained control (endurance) by the postural muscles. In fact, almost all daily living tasks require some degree of muscle and cardiopulmonary endurance.

Although strength and muscle endurance, as elements of muscle performance, are associated, they do not always correlate well with each other. Just because a muscle group is strong, it does not preclude the possibility that muscular endurance is impaired. For example, a strong worker has no difficulty lifting a 10-pound object several times, but does the worker have sufficient muscle endurance in the upper extremities and the stabilizing muscles of the trunk and lower extremities to lift 10-pound objects several hundred times during the course of a day's work without excessive fatigue or potential injury?

Endurance training. Endurance training (endurance exercise) is characterized by having a muscle contract and lift or lower a light load for many repetitions or sustain a muscle contraction for an extended period of time.^{9,10,192,210,260} The key parameters of endurance training are low-intensity muscle contractions, a large number of repetitions, and a prolonged time period. Unlike strength training, muscles adapt to endurance training by increases in their oxidative and metabolic capacities, which allows better delivery and use of oxygen. For many patients with impaired muscle performance, endurance training has a more positive impact on improving function than strength training. In addition, using low levels of resistance in an exercise program minimizes adverse forces on joints, produces less irritation to soft tissues, and is more comfortable than heavy resistance exercise.

Overload Principle

Description

The overload principle is a guiding principle of exercise prescription that has been one of the foundations on which the use of resistance exercise to improve muscle performance is based. Simply stated, if muscle performance is to improve, a load that exceeds the metabolic capacity of the muscle must be applied—that is, the muscle must be challenged to perform at a level greater than that to which it is accustomed.^{9,10,128,168,192,206} If the demands remain constant after the muscle has adapted, the level of muscle performance can be maintained but not increased.

Application of the Overload Principle

The overload principle focuses on the progressive loading of muscle by manipulating, for example, the intensity or volume of exercise. Intensity of resistance exercise refers to how much weight (resistance) is imposed on the muscle, whereas volume encompasses variables such as repetitions, sets, or frequency of exercise, any one or more of which can be gradually adjusted to increase the demands on the muscle.

• In a strength training program, the amount of resistance applied to the muscle is incrementally and progressively increased.

• For endurance training, more emphasis is placed on increasing the time a muscle contraction is sustained or the number of repetitions performed than on increasing resistance.

PRECAUTION: To ensure safety, the extent and progression of overload must always be applied in the context of the underlying pathology, age of the patient, stage of tissue healing, fatigue, and the overall abilities and goals of the patient. The muscle and related body systems must be given time to adapt to the demands of an increased load or repetitions before the load or number of repetitions is again increased.

SAID Principle

The SAID principle (specific adaptation to imposed demands)^9,192 suggests that a framework of specificity is a necessary foundation on which exercise programs should be built. This principle applies to all body systems and is an extension of Wolff's law (body systems adapt over time to the stresses placed on them). The SAID principle helps therapists determine the exercise prescription and which parameters of exercise should be selected to create specific training effects that best meet specific functional needs and goals.

Specificity of Training

Specificity of training, also referred to as specificity of exercise, is a widely accepted concept suggesting that the adaptive effects of training, such as improvement of strength, power, and endurance, are highly specific to the training method employed.^9,183 Whenever possible, exercises incorporated in a program should mimic the anticipated function. For example, if the desired functional activity requires greater muscular endurance than strength, the intensity and duration of exercises should be geared to improve muscular endurance.

Specificity of training also should be considered with respect to mode (type) and velocity of exercise^{24,80,205,225} as well as patient or limb position (joint angle)^161,162,285 and the movement pattern during exercise. For example, if the desired functional outcome is the ability to ascend and descend stairs, exercise should be performed eccentrically and concentrically in a weight-bearing pattern and progressed to the desired speed. Regardless of the simplicity or complexity of the motor task to be learned, task-specific practice always must be emphasized. It has been suggested that the basis of specificity of training is related to morphological and metabolic changes in muscle as well as neural adaptations to the training stimulus associated with motor learning.²¹⁸

Transfer of Training

In contrast to the SAID principle, carryover of training effects from one variation of exercise or task to another also has been reported. This phenomenon is called transfer of training, overflow, or a cross-training effect. Transfer of training has been reported to occur on a very limited basis with respect to the velocity of training^143,273 and the type or mode of exercise.⁸⁰ Furthermore, it has been suggested that a cross-training effect can occur from an exercised limb to a nonexercised, contralateral limb in a resistance training program.^283,284

A program of exercises designed to develop muscle strength also has been shown to improve muscular endurance at least moderately. In contrast, endurance training has little to no cross-training effect on strength.^9,18,103 Strength training at one speed of exercise has been shown to provide some improvement in strength at higher or lower speeds of exercise. However, the overflow effects are substantially less than the training effects resulting from specificity of training.

Despite the evidence that a small degree of transfer of training does occur in a resistance exercise program, most studies support the importance of designing an exercise program that most closely replicates the desired functional activities. As many variables as possible in the exercise program should match the requirements and demands placed on a patient during specific functional activities.

Reversibility Principle

Adaptive changes in the body's systems, such as increased strength or endurance, in response to a resistance exercise program are transient unless training-induced improvements are regularly used for functional activities or unless an individual participates in a maintenance program of resistance exercises.^{8,9,46,89,192}

Detraining, reflected by a reduction in muscle performance, begins within a week or two after the cessation of resistance exercises and continues until training effects are lost.^9,89,167,207 For this reason, it is imperative that gains in strength and endurance are incorporated into daily activities as early as possible in a rehabilitation program. It is also advisable for patients to participate in a maintenance program of resistance exercises as an integral component of a lifelong fitness program.

Knowledge of the factors that influence the force-producing capacity of normal muscle during an active contraction is fundamental to understanding how the neuromuscular system adapts as the result of resistance training. This knowledge, in turn, provides a basis on which a therapist is able to make sound clinical decisions when designing resistance exercise programs for patients with weakness and functional limitations as the result of injury or disease or to enhance physical performance and prevent or reduce the risk of injury in healthy individuals.

Factors that Influence Tension Generation in Normal Skeletal Muscle

NOTE: For a brief review of the structure of skeletal muscle, refer to Chapter 4 of this textbook. For in-depth information on muscle structure and function, numerous resources are available.^{182,183,192,210}

Diverse but interrelated factors affect the tension-generating capacity of normal skeletal muscle necessary to control the body and perform motor tasks. Determinants and correlates include morphological, biomechanical, neurological, metabolic, and biochemical factors. All contribute to the magnitude, duration, and speed of force production as well as how resistant or susceptible a muscle is to fatigue. Properties of muscle itself and as key neural factors and their impact on tension generation during an active muscle contraction are summarized in Table 6.1.^{9,182,183,192,210}

Additional factors—such as the energy stores available to muscle, the influence of fatigue and recovery from exercise, and a person's age, gender, and psychological/cognitive status, as well as many other factors—affect a muscle's ability to develop and sustain tension. A therapist must recognize that these factors affect a patient's performance during exercise as well as the potential outcomes of the exercise program.

Energy Stores and Blood Supply

Muscle needs adequate sources of energy (fuel) to contract, generate tension, and resist fatigue. The predominant fiber type found in the muscle and the adequacy of blood supply, which transports oxygen and nutrients to muscle and removes waste products, affect the tension-producing capacity of a muscle and its resistance to fatigue. The three main energy systems (ATP-PC system, anaerobic/glycolytic/lactic acid system, aerobic system) are reviewed in Chapter 7.

Fatigue

Fatigue is a complex phenomenon that affects muscle performance and must be considered in a resistance exercise program. Fatigue has a variety of definitions that are based on the type of fatigue being addressed.

Muscle (local) fatigue. Most relevant to resistance exercise is the phenomenon of skeletal muscle fatigue. Muscle (local) fatigue—the diminished response of muscle to a repeated stimulus—is reflected in a progressive decrement in the amplitude of motor unit potentials.^183,192 This occurs during exercise when a muscle repeatedly contracts statically or dynamically against an imposed load.

This acute physiological response to exercise is normal and reversible. It is characterized by a gradual decline in the force-producing capacity of the neuromuscular system—that is, a temporary state of exhaustion (failure), leading to a decrease in muscle strength.^{23,56,183,253}

The diminished response of the muscle is caused by a complex combination of factors, which include disturbances in the contractile mechanism of the muscle itself (associated with a decrease in energy stores, insufficient oxygen, reduced sensitivity and availability of intracellular calcium, and a build-up of H⁺) and perhaps reduced excitability at the neuromuscular junction or inhibitory (protective) influences from the central nervous system.^183,192,210

The fiber-type distribution of a muscle, which can be divided into two broad categories (type I and type II), affects how resistant it is to fatigue.^183,192,210 Type II (phasic, fast-twitch) muscle fibers are further divided into two additional classifications (types IIA and IIB) based on contractile and fatigue characteristics. Some resources subdivide type II fibers into three classifications.²³⁹ In general, type II fibers generate a great amount of tension within a short period of time, with type IIB being geared toward anaerobic metabolic activity and having a tendency to fatigue more quickly than type IIA fibers. Type I (tonic, slow-twitch) muscle fibers generate a low level of muscle tension but can sustain the contraction for a long time. These fibers are geared toward aerobic metabolism, as are type IIA fibers. However, type I fibers are more resistant to fatigue than type IIA. Table 6.2 compares the characteristics of muscle fiber types.^{9,182,183,210}

Because different muscles are composed of varying proportions of tonic and phasic fibers, their function becomes specialized. For example, a heavy distribution of type I (slow twitch, tonic) fibers is found in postural muscles, which allows muscles, such as the soleus, to sustain a low level of tension for extended periods of time to hold the body erect against gravity or stabilize against repetitive loads. On the other end of the fatigue spectrum, muscles with a large distribution of type IIB (fast twitch, phasic) fibers, such as the gastrocnemius or biceps brachii, produce a great burst of tension to enable a person to lift the entire body weight or to lift, lower, push, or pull a heavy load but fatigue quickly.

Clinical signs of muscular fatigue during exercise are summarized in Box 6.2.^192,210 When these signs and symptoms develop during resistance exercise, it is time to decrease the load on the exercising muscle or stop the exercise and shift to another muscle group to allow time for the fatigued muscle to rest and recover.

Cardiopulmonary (general) fatigue. This type of fatigue is the diminished response of an individual (the entire body) as the result of prolonged physical activity, such as walking, jogging, cycling, or repetitive lifting or digging. It is related to the body's ability to use oxygen efficiently. Cardiopulmonary fatigue associated with endurance training is probably caused by a combination of the following factors.^23,114

• Decrease in blood sugar (glucose) levels.

• Decrease in glycogen stores in muscle and liver.

• Depletion of potassium, especially in the elderly patient.

Threshold for fatigue. Threshold for fatigue is the level of exercise that cannot be sustained indefinitely.²³ A patient's threshold for fatigue could be noted as the length of time a contraction is maintained or the number of repetitions of an exercise that initially can be performed. This sets a baseline from which adaptive changes in physical performance can be measured.

Factors that influence fatigue. Factors that influence fatigue are diverse. A patient's health status, diet, or lifestyle (sedentary or active) all influence fatigue. In patients with neuromuscular, cardiopulmonary, inflammatory, cancer-related, or psychological disorders, the onset of fatigue is often abnormal.^4,56,98 For instance, it may occur abruptly, more rapidly, or at predictable intervals.

It is advisable for a therapist to become familiar with the patterns of fatigue associated with different diseases and medications. In multiple sclerosis, for example, the patient usually awakens rested and functions well during the early morning. By mid-afternoon, however, the patient reaches a peak of fatigue and becomes notably weak. Then, by early evening, the fatigue diminishes and strength returns. Patients with cardiac, peripheral vascular, and pulmonary diseases, as well as patients with cancer undergoing chemotherapy or radiation therapy, all have deficits that compromise the oxygen transport system. Therefore, these patients fatigue more readily and require a longer period of time for recovery from exercise.^4,98

Environmental factors, such as outside or room temperature, air quality, and altitude, also influence how quickly the onset of fatigue occurs and how much time is required for recovery from exercise.^169,192

Recovery from Exercise

Adequate time for recovery from fatiguing exercise must be built into every resistance exercise program. This applies to both intrasession and intersession recovery. After vigorous exercise, the body must be given time to restore itself to a state that existed prior to the exhaustive exercise. Recovery from acute exercise, in which the force-producing capacity of muscle returns to 90% to 95% of the pre-exercise capacity, usually takes 3 to 4 minutes, with the greatest proportion of recovery occurring in the first minute.^51,244

During recovery oxygen and energy stores are replenished quickly in muscles. Lactic acid is removed from skeletal muscle and blood within approximately 1 hour after exercise, and glycogen is replaced over several days.

Age

Muscle performance changes throughout the life span. Whether the goal of a resistance training program is to remediate impairments and activity limitations (functional limitations) or enhance fitness and performance of physical activities, an understanding of "typical" changes in muscle performance and response to exercise during each phase of life from early childhood through the advanced years of life is necessary to prescribe effective, safe resistance exercises for individuals of all ages. Key aspects of how muscle performance changes throughout life are discussed in this section and summarized in Box 6.3.

Early Childhood and Preadolescence

In absolute terms, muscle performance (specifically strength), which in part is related to the development of muscle mass, increases linearly with chronological age in both boys and girls from birth through early and middle childhood to puberty.^191,261,290 Muscle endurance also increases linearly during the childhood years.²⁹⁰ Muscle fiber number is essentially determined prior to or shortly after birth,²⁴¹ although there is speculation that fiber number may continue to increase into early childhood.²⁹⁰ The rate of fiber growth (increase in cross-sectional area) is relatively consistent from birth to puberty. Change in fiber type distribution is relatively complete by the age of 1, shifting from a predominance of type II fibers to a more balanced distribution of type I and type II fibers.²⁹⁰

Throughout childhood, boys have slightly greater absolute and relative muscle mass (kilograms of muscle per kilogram of body weight) than girls, with boys approximately 10% stronger than girls from early childhood to puberty.¹⁹¹ This difference may be associated with differences in relative muscle mass, although social expectations, especially by midchild-hood, also may contribute to the observed difference in muscle strength.

It is well established that an appropriately designed resistance exercise program can improve muscle strength in children above and beyond gains attributable to typical growth and development. Furthermore, training-induced strength gains in prepubescent children occur primarily as the result of neuromuscular adaptation—that is, without a significant increase in muscle mass.^22,91 Reviews of the literature^90,92 have cited many studies that support these findings. However, there is concern that children who participate in resistance training may be at risk for injuries, such as an epiphyseal fracture or avulsion fracture, because the musculoskeletal system is immature.^{22,27,103,266}

The American Academy of Pediatrics,⁶ the American College of Sports Medicine (ACSM),⁸ and the Centers for Disease Control and Prevention (CDC)³⁶ support youth participation in resistance training programs if they are designed appropriately, initiated at a reasonable age, and carefully supervised (Fig. 6.1). With this in mind, two important questions need to be addressed: (1) At what point during childhood is a resistance training program appropriate? and (2) What constitutes a safe training program?

There is general consensus that during the toddler, preschool, and even the early elementary school years, free play and organized but age-appropriate physical activities are effective methods to promote fitness and improve muscle performance, rather than structured resistance training programs. The emphasis throughout most or all of the first decade of life should be on recreation and learning motor skills.²⁷⁸

There is lack of agreement, however, on when and under what circumstances resistance training is an appropriate form of exercise for prepubescent children. Although age-appropriate physical activity on a regular basis has been recommended for children for some time,^6,8,36 during the past two or three decades it has become popular for older (preadolescent) boys and girls to participate in sport-specific training programs (including resistance exercises) before, during, and even after the season, in theory, to enhance athletic performance and reduce the risk of sport-related injury. Rehabilitation programs for prepubescent children who sustain injuries during everyday activities also may include resistance exercises. Consequently, an understanding of the effects of exercise in this age group must be the basis for establishing a safe program with realistic goals.

Adolescence

At puberty, as hormonal levels change, there is rapid acceleration in the development of muscle strength, especially in boys. During this phase of development, typical strength levels become markedly different in boys and girls, which, in part, are caused by hormonal differences between the sexes. Strength in adolescent boys increases about 30% per year between ages 10 and 16, with muscle mass peaking before muscle strength.^34,191 In adolescent girls, peak strength develops before peak weight.⁹⁵ Overall, during the adolescent years, muscle mass increases more than 5-fold in boys and approximately 3.5-fold in girls.^34,191 Although most longitudinal studies of growth stop at age 18, strength continues to develop, particularly in males, well into the second and even into the third decade of life.¹⁹¹

As with prepubescent children, resistance training during puberty also results in significant strength gains. During puberty, these gains average 30% to 40% above that which is expected as the result of normal growth and maturation.⁹⁰ A balanced training program for the adolescent involved in a sport often includes off-season and preseason aerobic conditioning and low-intensity resistance training followed by more vigorous, sport-specific training during the season.²² The benefits of strength training noted during puberty are similar to those that occur in prepubescent children.^89,93

Young and Middle Adulthood

Although data on typical strength and endurance levels during the second through the fifth decades of life are more often from studies of men than women, a few generalizations can be made that seem to apply to both sexes.¹⁸⁵ Strength reaches a maximal level earlier in women than men, with women reaching a peak during the second decade and in most men by age 30. Strength then declines approximately 1% per year,²⁹⁰ or 8% per decade.¹⁰⁷ This decline in strength appears to be minor until about age 50²⁷⁸ and tends to occur at a later age or slower rate in active adults versus those who are sedentary.^110,290 The potential for improving muscle performance with a resistance training program (Fig. 6.2 A and B) or by participation in even moderately demanding activities several times a week is high during this phase of life. Guidelines for young and middle-aged adults participating in resistance training as part of an overall fitness program have been published by the ACSM⁸ and the CDC.³⁵

Late Adulthood

The rate of decline in the tension-generating capacity of muscle in most cases accelerates to approximately 15% to 20% per decade in men and women in their sixties and seventies, and it increases to 30% per decade thereafter.^110,185 However, the rate of decline may be significantly less (only 0.3% decrease per year) in elderly men and women who maintain a high level of physical activity.¹¹⁸ These disparate findings and others suggest that loss of muscle strength during the advanced years may be due, in part, to progressively greater inactivity and disuse.³⁹ Loss of muscle in the lower extremity and trunk strength and stability during late adulthood—most notably during the seventies, eighties, and beyond—is associated with a gradual deterioration of functional abilities and an increase in the frequency of falling.^39,130

The decline in muscle strength and endurance in the elderly is associated with many factors in addition to progressive disuse and inactivity. It is difficult to determine if these factors are causes or effects of an age-related decrease in strength. Neuromuscular factors include a decrease in muscle mass (atrophy), decrease in the number of type I and II muscle fibers with a corresponding increase in connective tissue in muscle, a decrease in the cross-sectional size of muscle, selective atrophy of type II fibers, and change in the length-tension relationship of muscle associated with loss of flexibility, more so than deficits in motor unit activation and firing rate.^{35,107,141,239,271,278,295} The decline in the number of motor units appears to begin after age 60.¹⁴¹ All of these changes have an impact on strength and physical performance.

In addition to decreases in muscle strength, declines in the speed of muscle contraction, muscle endurance, and the ability to recover from muscular fatigue occur with advanced age.^141,271 The time needed to produce the same absolute and relative levels of torque output and the time necessary to achieve relaxation after a voluntary contraction are lengthened in the elderly compared to younger adults.¹⁰⁷ Consequently, as velocity of movement declines, so does the ability to generate muscle power during activities that require quick responses, such as rising from a low chair or making balance adjustments to prevent a fall. Deterioration of muscle power with age has a stronger relationship to functional limitations and disability than does muscle strength.²²⁹

Information on changes in muscle endurance with aging is limited. There is some evidence to suggest that the ability to sustain low-intensity muscular effort also declines with age, in part because of reduced blood supply and capillary density in muscle, decreased mitochondrial density, changes in enzymatic activity level, and decreased glucose transport.¹⁰⁷ As a result, muscle fatigue may tend to occur more readily in the elderly. In the healthy and active (community-dwelling) elderly population, the decline in muscle endurance appears to be minimal well into the seventies.¹⁴¹

During the past few decades, as the health care community and the public have become more aware of the benefits of resistance training during late adulthood, a greater number of older adults are participating in fitness programs that include resistance exercises (Fig. 6.3). ACSM and the CDC have published guidelines for resistance training for healthy adults over 60 to 65 years of age (see Box 6.18).^8,37

Psychological and Cognitive Factors

An array of psychological factors can positively or negatively influence muscle performance and how easily, vigorously, or cautiously a person moves. Just as injury and disease adversely affect muscle performance, so can a person's mental status. For example, fear of pain, injury, or re-injury, depression related to physical illness, or impaired attention or memory as the result of age, head injury, or the side effects of medication can adversely affect the ability to develop or sustain sufficient muscle tension for execution or acquisition of functional motor tasks. In contrast, psychological factors can positively influence physical performance.

The principles and methods employed to maximize motor performance and learning as functions of effective patient education are discussed in Chapter 1. These principles and methods should be applied in a resistance training program to develop a requisite level of muscle strength, power, and endurance for functional activities. The following interrelated psychological factors as well as other aspects of motor learning may influence muscle performance and the effectiveness of a resistance training program.

Attention

A patient must be able to focus on a given task (exercise) to learn how to perform it correctly. Attention involves the ability to process relevant data while screening out irrelevant information from the environment and to respond to internal cues from the body. Both are necessary when first learning an exercise and later when carrying out an exercise program independently. Attention to form and technique during resistance training is necessary for patient safety and optimal long-term training effects.

Motivation and Feedback

If a resistance exercise program is to be effective, a patient must be willing to put forth and maintain sufficient effort and adhere to an exercise program over time to improve muscle performance for functional activities. Use of activities that are meaningful and are perceived as having potential usefulness or periodically modifying an exercise routine help maintain a patient's interest in resistance training. Charting or graphing a patient's strength gains, for example, also helps sustain motivation. Incorporating gains in muscle performance into functional activities as early as possible in a resistance exercise program puts improvements in strength to practical use, thereby making those improvements meaningful.

The importance of feedback for learning an exercise or a motor skill is discussed in Chapter 1. In addition, feedback can have a positive impact on a patient's motivation and subsequent adherence to an exercise program. For example, some computerized equipment, such as isokinetic dynamometers, provide visual or auditory signals that let the patient know if each muscle contraction during a particular exercise is in a zone that causes a training effect. Documenting improvements over time, such as the amount of weight (exercise load) used during various exercises or changes in walking distance or speed, also provides positive feedback to sustain a patient's motivation in a resistance exercise program.

Physiological Adaptations to Resistance Exercise

The use of resistance exercise in rehabilitation and conditioning programs has a substantial impact on all systems of the body. Resistance training is equally important for patients with impaired muscle performance and individuals who wish to improve or maintain their level of fitness, enhance performance, or reduce the risk of injury. When body systems are exposed to a greater than usual but appropriate level of resistance in an exercise program, they initially react with a number of acute physiological responses and then later adapt—that is, body systems accommodate over time to the newly imposed physical demands.^8,9,192 Training-induced adaptations to resistance exercise, known as chronic physiological responses, that affect muscle performance are summarized in Table 6.3 and discussed in this section. Key differences in adaptations from strength training versus endurance training are noted.

Adaptations to overload create changes in muscle performance and, in part, determine the effectiveness of a resistance training program. The time course for these adaptations to occur varies from one individual to another and is dependent on a person's health status and previous level of participation in a resistance exercise program.¹⁰

Neural Adaptations

It is well accepted that in a resistance training program the initial, rapid gain in the tension-generating capacity of skeletal muscle is attributed largely to neural responses, not adaptive changes in muscle itself.^{109,183,203,235} This is reflected by an increase in electromyographic (EMG) activity during the first 4 to 8 weeks of training with little to no evidence of muscle fiber hypertrophy. It is also possible that increased neural activity is the source of additional gains in strength late in a resistance training program even after muscle hypertrophy has reached a plateau.^168,192

Neural adaptations are attributed to motor learning and improved coordination^{109,167,169,192} and include increased recruitment in the number of motor units firing as well as an increased rate and synchronization of firing.^{109,167,225,235} It is speculated that these changes are caused by a decrease in the inhibitory function of the central nervous system (CNS), decreased sensitivity of the Golgi tendon organ (GTO), or changes at the myoneural junction of the motor unit.^109,235

Skeletal Muscle Adaptations

Hypertrophy

As noted previously, the tension-producing capacity of muscle is directly related to the physiological cross-sectional area of the individual muscle fibers. Hypertrophy is an increase in the size (bulk) of an individual muscle fiber caused by an increase in myofibrillar volume.^206,271 After an extended period of moderate- to high-intensity resistance training, usually by 4 to 8 weeks^1,287 but possibly as early as 2 to 3 weeks with very high-intensity resistance training,²⁵⁵ hypertrophy becomes an increasingly important adaptation that accounts for strength gains in muscle.

Although the mechanism of hypertrophy is complex and the stimulus for growth is not clearly understood, hypertrophy of skeletal muscle appears to be the result of an increase in protein (actin and myosin) synthesis and a decrease in protein degradation. Hypertrophy is also associated with biochemical changes that stimulate uptake of amino acids.^{167,192,206,271}

The greatest increases in protein synthesis and, therefore, hypertrophy are associated with high-volume, moderate-resistance exercise performed eccentrically.^167,232 Furthermore, it is the type IIB muscle fibers that appear to increase in size most readily with resistance training.^192,210

Hyperplasia

Although the topic has been debated for many years and evidence of the phenomenon is sparse, there is some thought that a portion of the increase in muscle size that occurs with heavy resistance training is caused by hyperplasia, an increase in the number of muscle fibers. It has been suggested that this increase in fiber number, observed in laboratory animals,^116,117 is the result of longitudinal splitting of fibers.^15,139,199 It has been postulated that fiber splitting occurs when individual muscle fibers increase in size to a point at which they are inefficient, then subsequently split to form two distinct fibers.¹¹⁶

Critics of the concept of hyperplasia suggest that evidence of fiber splitting actually may be caused by inappropriate tissue preparation in the laboratory.¹¹⁵ The general opinion in the literature is that hyperplasia either does not occur, or if it does occur to a slight degree, its impact is insignificant.^183,188 In a review article published in the late 1990s, it was the authors' opinion that if hyperplasia is a valid finding, it probably accounts for a very small proportion (less than 5%) of the increase in muscle size that occurs with resistance training.¹⁶⁹

Muscle Fiber Type Adaptation

As previously mentioned, type II (phasic) muscle fibers preferentially hypertrophy with heavy resistance training. In addition, a substantial degree of plasticity exists in muscle fibers with respect to contractile and metabolic properties.²³⁹ Transformation of type IIB to type IIA is common with endurance training,²³⁹ as well as during the early weeks of heavy resistance training,²⁵⁵ making the type II fibers more resistant to fatigue. There is some evidence that demonstrates type I to type II fiber type conversion in the denervated limbs of laboratory animals,^216,302 in humans with spinal cord injury, and after an extended period of weightlessness associated with space flight.²³⁹ However, there is little to no evidence of type II to type I conversion under training conditions in rehabilitation or fitness programs.^192,239

Vascular and Metabolic Adaptations

Adaptations of the cardiovascular and respiratory systems as the result of low-intensity, high-volume resistance training are discussed in Chapter 7. Opposite to what occurs with endurance training, when muscles hypertrophy with high-intensity, low-volume training, capillary bed density actually decreases because of an increase in the number of myofilaments per fiber.⁹ Athletes who participate in heavy resistance training actually have fewer capillaries per muscle fiber than endurance athletes and even untrained individuals.^154,269 Other changes associated with metabolism, such as a decrease in mitochondrial density, also occur with high- intensity resistance training.^9,167 This is associated with reduced oxidative capacity of muscle.

Adaptations of Connective Tissues

Although the evidence is limited, it appears that the tensile strength of tendons and ligaments as well as bone increases with resistance training designed to improve the strength or power of muscles.^49,259,303

Tendons, Ligaments, and Connective Tissue in Muscle

Strength improvement in tendons probably occurs at the musculotendinous junction, whereas increased ligament strength may occur at the ligament-bone interface. It is believed that tendon and ligament tensile strength increases in response to resistance training to support the adaptive strength and size changes in muscle.³⁰³ The connective tissue in muscle (around muscle fibers) also thickens, giving more support to the enlarged fibers.¹⁹² Consequently, strong ligaments and tendons may be less prone to injury. It is also thought that noncontractile soft tissue strength may develop more rapidly with eccentric resistance training than with other types of resistance exercises.^258,259

Bone

Numerous sources indicate there is a high correlation between muscle strength and the level of physical activity across the life span with bone mineral density.²³⁸ Consequently, physical activities and exercises, particularly those performed in weight-bearing positions, are typically recommended to minimize or prevent age-related bone loss.²²⁶ They are also prescribed to reduce the risk of fractures or improve bone density when osteopenia or osteoporosis is already present.^54,238

Research continues to determine the most effective forms of exercise to enhance bone density and prevent age-related bone loss and fractures. For additional information on prevention and management of osteoporosis, refer to Chapter 11.

Determinants of Resistance Exercise

Many elements (variables) determine whether a resistance exercise program is appropriate, effective, and safe. This holds true when resistance training is a part of a rehabilitation program for individuals with known or potential impairments in muscle performance or when it is incorporated into a general conditioning program to improve the level of fitness of healthy individuals.

Each of the interrelated elements noted in Box 6.4 and discussed in this section should be addressed to improve one or more aspects of muscle performance and achieve desired functional outcomes. Appropriate alignment and stabilization are always basic elements of any exercise designed to improve muscle performance. A suitable dosage of exercise must also be determined. In resistance training, dosage includes intensity, volume, frequency, and duration of exercise and rest interval. Each factor is a mechanism by which the muscle can be progressively overloaded to improve muscle performance. The velocity of exercise and the mode(type) of exercise must also be considered. ACSM denotes the key determinants of a resistance training program by the acronym, FITT, which represents frequency, intensity, time, and type of exercise.⁸

Consistent with the SAID principle discussed in the first section of this chapter, these elements of resistance exercise must be specific to the patient's desired functional goals.

Other factors, such as the underlying cause or causes of the deficits in muscle performance, the extent of impairment, and the patient's age, medical history, mental status, and social situation also affect the design and implementation of a resistance exercise program.

Alignment and Stabilization

Just as correct alignment and effective stabilization are basic elements of manual muscle testing and dynamometry, they are also crucial in resistance exercise. To strengthen a specific muscle or muscle group effectively and avoid substitute motions, appropriate positioning of the body and alignment of a limb or body segment are essential. Substitute motions are compensatory movement patterns caused by muscle action of a stronger adjacent agonist or a muscle group that normally serves as a stabilizer (fixator).¹⁵⁸ If the principles of alignment and stabilization for manual muscle testing^137,158 are applied whenever possible during resistance exercise, substitute motions can usually be avoided.

Alignment

Alignment and muscle action. Proper alignment is determined by the direction of muscle fibers and the line of pull of the muscle to be strengthened. The patient or a body segment must be positioned so the direction of movement of a limb or segment of the body replicates the action of the muscle or muscle groups to be strengthened. For example, to strengthen the gluteus medius, the hip must remain slightly extended, not flexed, and the pelvis must be rotated slightly forward as the patient abducts the lower extremity against the applied resistance. If the hip is flexed as the leg abducts, the adjacent tensor fasciae latae becomes the prime mover and is strengthened.

Alignment and gravity. The alignment or position of the patient or the limb with respect to gravity also may be important during some forms of resistance exercises, particularly if body weight or free weights (dumbbells, barbells, cuff weights) are the source of resistance. The patient or limb should be positioned so the muscle being strengthened acts against the resistance of gravity and the weight. This, of course, is contingent on the comfort and mobility of the patient.

Staying with the example of strengthening the gluteus medius, if a cuff weight is placed around the lower leg, the patient must assume the side-lying position so abduction occurs through the full ROM against gravity and the additional resistance of the cuff weight. If the patient rolls toward the supine position, the resistance force is applied primarily to the hip flexors, not the abductors.

Stabilization

Stabilization refers to holding down a body segment or holding the body steady.¹⁵⁸ To maintain appropriate alignment, ensure the correct muscle action and movement pattern, and avoid unwanted substitute motions during resistance exercise, effective stabilization is imperative. Exercising on a stable surface, such as a firm treatment table, helps hold the body steady. Body weight also may provide a source of stability during exercise, particularly in the horizontal position. It is most common to stabilize the proximal attachment of the muscle being strengthened, but sometimes the distal attachment is stabilized as the muscle contracts.

Stabilization can be achieved externally or internally.

• External stabilization can be applied manually by the therapist or sometimes by the patient with equipment, such as belts and straps, or by a firm support surface, such as the back of a chair or the surface of a treatment table.

• Internal stabilization is achieved by an isometric contraction of an adjacent muscle group that does not enter into the movement pattern but holds the body segment of the proximal attachment of the muscle being strengthened firmly in place. For example, when performing a bilateral straight leg raise, the abdominals contract to stabilize the pelvis and lumbar spine as the hip flexors raise the legs. This form of stabilization is effective only if the fixating muscle group is strong enough or not fatigued.

Intensity

The intensity of exercise in a resistance training program is the amount of resistance (weight) imposed on the contracting muscle during each repetition of an exercise. The amount of resistance is also referred to as the exercise load (training load)—that is, the extent to which the muscle is loaded or how much weight is lifted, lowered, or held.

Remember, consistent with the overload principle, to improve muscle performance the muscle must be loaded to an extent greater than loads usually incurred. One way to overload a muscle progressively is to gradually increase the amount of resistance used in the exercise program.^{8,10,103,168,169} The intensity of exercise and the degree to which the muscle is overloaded is also dependent on the volume, frequency, and order of exercise or the length of rest intervals.

Submaximal Versus Maximal Exercise Loads

Many factors, such as the goals and expected functional outcomes of the exercise program; the cause of deficits in muscle performance; the extent of impairment; the stage of healing of injured tissues; the patient's age, general health, and fitness level; and other factors, determine whether the exercise is carried out against submaximal or maximal muscle loading. In general, the level of resistance is often lower in rehabilitation programs for persons with impairments than in conditioning programs for healthy individuals.

Indications for submaximal loading for moderate to low-intensity exercise versus near-maximal or maximal loading for high-intensity exercise are summarized in Table 6.4.

PRECAUTION: The intensity of exercise should never be so great as to cause pain. As the intensity of exercise increases and a patient exerts a maximal or near-maximal effort, cardiovascular risks increase substantially. A patient needs to be continually reminded to incorporate rhythmic breathing into each repetition of an exercise to minimize these risks.

Initial Exercise Load (Amount of Resistance) and Documentation of Training Effects

It is always challenging to estimate how much resistance to apply manually or how much weight a patient should use during resistance exercises to improve muscle strength particularly at the beginning of a strengthening program. With manual resistance exercise, the decision is entirely subjective, based on the therapist's judgment during exercise. In an exercise program using mechanical resistance, the determination can be made quantitatively.

Repetition Maximum

One method of measuring the effectiveness of a resistance exercise program and calculating an appropriate exercise load for training is to determine a repetition maximum. This term was first reported decades ago by DeLorme in his investigations of an approach to resistance training called progressive resistive exercise (PRE).^64,65 A repetition maximum (RM) is defined as the greatest amount of weight (load) a muscle can move through the full, available ROM with control a specific number of times before fatiguing.

Use of a repetition maximum. There are two main reasons for determining a repetition maximum: (1) to document a baseline measurement of the dynamic strength of a muscle or muscle group against which exercise-induced improvements in strength can be compared; and (2) to identify an initial exercise load (amount of weight) to be used during exercise for a specified number of repetitions. DeLorme reported use of a 1-RM (the greatest amount of weight a subject can move through the available ROM just one time) as the baseline measurement of a subject's maximum effort but used a multiple RM, specifically a 10-RM, (the amount of weight that could be lifted and lowered 10 times through the ROM) during training.⁶⁵

Despite criticism that establishing a 1-RM involves some trial and error, it is a frequently used method for measuring muscle strength in research studies and has been shown to be a safe and reliable measurement tool with healthy young adults and athletes^103,169 as well as active older adults prior to beginning conditioning programs.^202,265,276

PRECAUTION: Use of a 1-RM as a baseline measurement of dynamic strength is inappropriate for some patient populations because it requires one maximum effort. It is not safe for patients, for example, with joint impairments, patients who are recovering from or who are at risk for soft tissue injury, or patients with known or at risk for osteoporosis or cardiovascular pathology.

Alternative Methods of Determining Baseline Strength or an Initial Exercise Load

Cable tensiometry¹⁹² and isokinetic or handheld dynamometry⁵⁷ are alternatives to a repetition maximum for establishing a baseline measurement of dynamic or static strength. A percentage of body weight also has been proposed to estimate how much resistance (load) should be used in a strength training program.²³⁶ Some examples for several exercises are noted in Box 6.5. The percentages indicated are meant as guidelines for the advanced stage of rehabilitation and are based on 10 repetitions of each exercise at the beginning of an exercise program. Percentages vary for different muscle groups.

When a maximum effort is inappropriate, the level of perceived loading, as measured by the Borg CR 10 Scale, has been shown to be a useful tool in estimating an appropriate level of resistance and sufficient exercise intensity for muscle strengthening.¹³

Training Zone

After establishing the baseline RM, the amount of resistance (exercise load) to be used at the initiation of resistance training often is calculated as a percentage of a 1-RM for a particular muscle group. At the beginning of an exercise program the percentage necessary to achieve training-induced adaptations in strength is low (30% to 40%) for sedentary, untrained individuals or very high (>80%) for those already highly trained. For healthy but untrained adults, a typical training zone usually falls between 40% and 70% of the baseline RM.^8,10,14 The lower percentage of this range is safer at the beginning of a program to enable an individual to focus on learning correct exercise form and technique before progressing the exercise load to 60% to 70%.

Exercising at a low to moderate percentage of the established RM is recommended for children and the elderly.^8,10 For patients with significant deficits in muscle strength or to train for muscular endurance, using a low load—possibly at the 30% to 50% level—is safe yet challenging.

Volume

In resistance training the volume of exercise is the summation of the total number of repetitions and sets of a particular exercise during a single exercise session times the intensity of the exercise.^8,10,168 The same combination of repetitions and sets is not and should not be used for all muscle groups.

There is an inverse relationship between the sets and repetitions of an exercise and the intensity of the resistance. The higher the intensity (load), the lower the number of repetitions and sets possible. Conversely, the lower the load, the greater the number of repetitions and sets possible. Therefore, the exercise load directly dictates how many repetitions and sets are possible.

Repetitions. The number of repetitions in a dynamic exercise program refers to the number of times a particular movement is repeated. More specifically, it is the number of muscle contractions performed to move the limb through a series of continuous and complete excursions against a specific exercise load.

If the RM designation is used, the number of repetitions at a specific exercise load is reflected in the designation. For example, 10 repetitions at a particular exercise load is a 10-RM. If a 1-RM has been established as a baseline level of dynamic strength, a percentage of the 1-RM used as the exercise load influences the number of repetitions a patient is able to perform before fatiguing. The "average," untrained adult, when exercising with a load that is equivalent to 75% of the 1-RM, is able to complete approximately 10 repetitions before needing to rest.^18,192 At 60% intensity about 15 repetitions are possible, and at 90% intensity only 4 or 5 repetitions are usually possible.

For practical reasons, after a beginning exercise load is selected, the target number of repetitions performed for each exercise before a brief rest is often within a range rather than an exact number of repetitions. For example, a patient might be able to complete between 8 and 10 repetitions against a specified load before resting. This is sometimes referred to as a RM zone,¹⁹² it gives the patient a goal but builds in some flexibility.

The number of repetitions selected depends on the patient's status and whether the goal of the exercise is to improve muscle strength or endurance. No optimal number for strength training or endurance training has been identified. Training effects (greater strength) have been reported employing a 2- to 3-RM to a 15-RM.^18,170

Sets. A predetermined number of consecutive repetitions grouped together is known as a set or bout of exercise. After each set of a specified number of repetitions, there is a brief interval of rest. For example, during a single exercise session to strengthen a particular muscle group, a patient might be directed to lift an exercise load 8 to 10 times, rest, and then lift the load another 8 to 10 times. That would be two sets of an 8- to 10-RM.

As with repetitions, there is no optimal number of sets per exercise session, but 2 to 4 sets is a common recommendation for adults.⁸ As few as one set and as many as six sets, however, have yielded positive training effects.^10,168 Single-set exercises at low intensities are most common in the very early phases of a resistance exercise program or in a maintenance program. Multiple-set exercises are used to progress the program and have been shown to be superior to single-set regimens in advanced training.¹⁷⁰

Training to Improve Strength or Endurance: Impact of Exercise Load and Repetitions

Overall, because many variations of intensity and volume cause positive training-induced adaptations in muscle performance, there is a substantial amount of latitude for selecting an exercise load/repetition and set scheme for each exercise. The question becomes: Is the goal to improve strength, power, or muscular endurance?

To Improve Muscle Strength

In DeLorme's early studies,^64,65 three sets of a 10-RM performed for 10 repetitions over the training period led to gains in strength. Current recommendations for strength training vary somewhat. One resource¹⁴ suggests that a threshold of 40% to 60% of maximum effort is necessary for adaptive strength gains to occur in a healthy but untrained individual. However, other resources recommend using a moderate exercise load (60% to 80% of a 1-RM) that causes fatigue after 8 to 12 repetitions for 2 or 3 sets.^8,168 When fatigue no longer occurs after the target number of repetitions has been completed, the level of resistance is increased to overload the muscle once again.

To Improve Muscle Endurance

Training to improve muscle (local) endurance involves performing many repetitions of an exercise against a submaximal load.^9,168,260 For example, as many as three to five sets of 40 to 50 repetitions against a low amount of weight or a light grade of elastic resistance might be used. When increasing the number of repetitions or sets becomes inefficient, the load can be increased slightly.

Endurance training also can be accomplished by maintaining an isometric muscle contraction for incrementally longer periods of time. Because endurance training is performed against very low levels of resistance, it can and should be initiated very early in a rehabilitation program without risk of injury to healing tissues.

Exercise Order

The sequence in which exercises are performed during an exercise session has an impact on muscle fatigue and adaptive training effects. When several muscle groups are exercised in a single session, as is the case in most rehabilitation or conditioning programs, large muscle groups should be exercised before small muscle groups, and multi-joint exercises should be performed before single-joint exercises.^{10,103,167,168} In addition, after an appropriate warm-up, higher intensity exercises should be performed before lower intensity exercises.¹⁰

Frequency

Frequency in a resistance exercise program refers to the number of exercise sessions per day or per week.^8,10 Frequency also may refer to the number of times per week specific muscle groups are exercised or certain exercises are performed.^8,168 As with other aspects of dosage, frequency is dependent on other determinants, such as intensity and volume as well as the patient's goals, general health status, previous participation in a resistance exercise program, and response to training. The greater the intensity and volume of exercise, the more time is needed between exercise sessions to recover from the temporarily fatiguing effects of exercise. A common cause of a decline in performance from overtraining (see discussion later in the chapter) is excessive frequency, inadequate rest intervals, and progressive fatigue.

Some forms of exercise should be performed less frequently than others because they require greater recovery time. It has been known for some time that high-intensity eccentric exercise, for example, is associated with greater microtrauma to soft tissues and a higher incidence of delayed-onset muscle soreness than concentric exercise.^16,106,212 Therefore, rest intervals between exercise sessions are longer and the frequency of exercise is less than with other forms of exercise.

Although an optimal frequency per week has not been determined, a few generalizations can be made. Initially in an exercise program, short sessions of exercises sometimes can be performed on a daily basis several times per day as long as the intensity of exercise and number of repetitions are low. This frequency often is indicated for early postsurgical patients when the operated limb is immobilized and the extent of exercise is limited to nonresisted isometric (setting) exercises to minimize the risk of muscle atrophy. As the intensity and volume of exercise increases, a frequency of 2 to 3 times per week, every other day, or up to five exercise sessions per week is common.^8,10,103,167 A rest interval of 48 hours for training major muscle groups can be achieved by exercising the upper extremities one day and the lower extremities during the next exercise session.

Frequency is again reduced for a maintenance program, usually to two times per week. With prepubescent children and the very elderly, frequency typically is limited to no more than 2 to 3 sessions per week.^8,10,36,37 Highly trained athletes involved in body building, power lifting, and weight lifting, who know their own response to exercise, often train at a high-intensity and volume up to 6 days per week.^10,168,170

Duration

Exercise duration is the total number of weeks or months during which a resistance exercise program is carried out. Depending on the cause of impaired muscle performance, some patients require only a month or two of training to return to the desired level of function or activity, whereas others need to continue the exercise program for a lifetime to maintain optimal function.

As noted earlier in the chapter, strength gains observed early in a resistance training program (after 2 to 3 weeks) primarily are the result of neural adaptation. For significant changes to occur in muscle, such as hypertrophy or increased vascularization, at least 6 to 12 weeks of resistance training is required.^1,8,192

Rest Interval (Recovery Period)

Purpose of rest intervals. Rest is a critical element of a resistance training program and is necessary to allow time for the body to recuperate from the acute effects of exercise associated with muscle fatigue or to offset adverse responses, such as exercise-induced, delayed-onset muscle soreness. Only with an appropriate balance of progressive loading and adequate rest intervals can muscle performance improve. Therefore, rest between sets of exercise and between exercise sessions must be addressed.

Integration of rest into exercise. Rest intervals for each exercising muscle group are dependent on the intensity and volume of exercise. In general, the higher the intensity of exercise the longer the rest interval. For moderate-intensity resistance training, a 2- to 3-minute rest period after each set is recommended. A shorter rest interval is adequate after low-intensity exercise. Longer rest intervals (>3 minutes) are appropriate with high-intensity resistance training, particularly when exercising large, multi-joint muscles, such as the hamstrings, which tend to fatigue rapidly.^8,10 While the muscle group that was just exercised is resting, resistance exercises can be performed by another muscle group in the same extremity or by the same muscle group in the opposite extremity.

Patients with pathological conditions that make them more susceptible to fatigue, as well as children and the elderly, should rest at least 3 minutes between sets by performing a nonresisted exercise, such as low-intensity cycling, or performing the same exercise with the opposite extremity. Remember, active recovery is more efficient than passive recovery for neutralizing the effects of muscle fatigue.

Rest between exercise sessions must also be considered. When strength training is initiated at moderate intensities (typically in the intermediate phase of a rehabilitation program after soft tissue injury), a 48-hour rest interval between exercise sessions (that is, training every other day) allows the patient adequate time for recovery.

Mode of Exercise

The mode of exercise in a resistance exercise program refers to the form of exercise, the type of muscle contraction that occurs, and the manner in which the exercise is carried out. For example, a patient may perform an exercise dynamically or statically or in a weight-bearing or nonweight-bearing position. Mode of exercise also encompasses the form of resistance—that is, how the exercise load is applied. Resistance can be applied manually or mechanically.

As with other determinants of resistance training, the modes of exercise selected are based on a multitude of factors already highlighted throughout this section. A brief overview of the various modes of exercise is presented in this section. An in-depth explanation and analysis of each of these types of exercise can be found in the next section of this chapter and in Chapter 7.

Type of Muscle Contraction

Figure 6.4 depicts the types of muscle contraction that may be performed in a resistance exercise program and their relationships to each other and to muscle performance.^182,210,248

• Isometric (static) or dynamic muscle contractions are two broad categories of exercise.

• Dynamic resistance exercises can be performed using concentric (shortening) or eccentric (lengthening) contractions, or both.

• When the velocity of limb movement is held consistent by a rate-controlling device, the term isokinetic contraction is sometimes used.²⁴⁸ An alternative perspective is that this is simply a dynamic (shortening or lengthening) contraction that occurs under controlled conditions.¹⁸²

Position for Exercise: Weight-Bearing or Nonweight-Bearing

The patient's body position or the position of a limb in nonweight-bearing or weight-bearing positions also alters the mode of exercise. When a nonweight-bearing position is assumed and the distal segment (foot or hand) moves freely during exercise, the term open-chain exercise (or a variation of this term) is often used. When a weight-bearing position is assumed and the body moves over a fixed distal segment, the term closed-chain exercise is commonly used.^182,210,248 Concepts and issues associated with the use of this terminology are addressed later in this chapter.

Forms of Resistance

• Manual resistance and mechanical resistance are the two broad methods by which resistance can be applied.

• A constant or variable load can be imposed using mechanical resistance (e.g., free weights or weight machines).

• Accommodating resistance¹³⁸ can be implemented by use of an isokinetic dynamometer that controls the velocity of active movement during exercise.

• Body weight or partial body weight is also a source of resistance if the exercise occurs in an antigravity position. Although an exercise performed against only the resistance of the weight of a body segment (and no additional external resistance) is defined as an active rather than an active-resistive exercise, a substantial amount of resistance from the weight of the body can be imposed on contracting muscles by altering a patient's position. For example, progressive loads can be placed on upper extremity musculature during push-ups by starting with wall push-ups while standing, progressing to push-ups while leaning against a countertop, push-ups in a horizontal position (Fig. 6.5), and finally push-ups from a head-down position over a large exercise ball.

Energy Systems

Modes of exercise also can be classified by the energy systems used during the exercise. Anaerobic exercise involves high-intensity (near-maximal) exercise carried out for a very few number of repetitions because muscles rapidly fatigue. Strengthening exercises fall into this category. Aerobic exercise is associated with low-intensity, repetitive exercise of large muscle groups performed over an extended period of time. This mode of exercise primarily increases muscular and cardiopulmonary endurance (refer to Chapter 7 for an in-depth explanation).

Range of Movement: Short-Arc or Full-Arc Exercise

Resistance through the full, available range of movement (full-arc exercise) is necessary to develop strength through the ROM. Sometimes resistance exercises are executed through only a portion of the available range. This is known as short-arc exercise. This form of exercise is used to avoid a painful arc of motion or a portion of the range in which the joint is unstable or to protect healing tissues after injury or surgery.

Mode of Exercise and Application to Function

Mode-specific training is essential if a resistance training program is to have a positive impact on function. When tissue healing allows, the type of muscle contractions performed or the position in which an exercise is carried out should mimic the desired functional activity as closely as possible.²⁰⁵

Velocity of Exercise

The velocity at which a muscle contracts significantly affects the tension that the muscle produces and subsequently affects muscular strength and power.²¹⁷ The velocity of exercise is frequently manipulated in a resistance training program to prepare the patient for a variety of functional activities that occur across a wide spectrum of slow to fast velocities.

Force-Velocity Relationship

The force-velocity relationship is different during concentric and eccentric muscle contractions, as depicted in Figure 6.6.

Concentric Muscle Contraction

During a maximum effort concentric muscle contraction, as the velocity of muscle shortening increases, the force the muscle can generate decreases. EMG activity and torque decrease as a muscle shortens at faster contractile velocities, possibly because the muscle may not have sufficient time to develop peak tension.^{53,182,210,248,294}

Eccentric Muscle Contraction

Findings are less consistent for eccentric than concentric muscle actions. During a maximum effort eccentric contraction, as the velocity of active muscle lengthening increases, force production in the muscle initially increases to a point but then quickly levels off.^{38,63,182,210,248} The initial increase in force production may be a protective response of the muscle when it is first overloaded. It is thought that this increase may be important for shock absorption or rapid deceleration of a limb during quick changes of direction.^78,248 The rise in tension also may be caused by stretch of the noncontractile tissue in muscle.⁶³ In contrast, other research indicates that eccentric force production is essentially unaffected by velocity and remains constant at slow and fast velocities.^53,121

Application to Resistance Training

A range of slow to fast exercise velocities has a place in an exercise program. Resistance training with free weights is safe and effective only at slow to medium velocities of limb movement so the patient can maintain control of the moving weight. Because many functional activities involve reasonably fast velocities of limb movement, training at only slow velocities is inadequate. The development of the isokinetic dynamometer during the late 1960s^138,200 gave clinicians a tool to implement resistance training at fast as well as slow velocities. In recent years, some variable resistance exercise units (pneumatic and hydraulic) and elastic resistance products also have afforded additional options for safely training at fast velocities.

Velocity-specific training is fundamental to a successful rehabilitation program. Results of numerous studies since the 1970s have shown that training-induced strength gains in a resistance exercise program primarily occur at the training velocities,^24,80,149 with limited transfer of training (physiological overflow) above and below the training velocities.^143,273 Accordingly, training velocities for resistance exercises should be geared to match or approach the demands of the desired functional activities.^57,149

Isokinetic training, using velocity spectrum rehabilitation regimens, and plyometric training, also known as stretch-shortening drills, often emphasize high-speed training. These approaches to exercise are discussed later in this chapter and in Chapter 23, respectively.

Periodization and Variation of Training

Periodization, also known as periodized training, is an approach to resistance training that breaks up a training program into periods and builds systematic variation in exercise intensity and repetitions, sets, or frequency at regular intervals over a specified period of time.^102,168 This approach to training was developed for highly trained athletes preparing for competitive weight lifting or power lifting events. The concept was designed for optimal progression of training programs, to prevent over-training and psychological staleness prior to competition, and to optimize performance during competition.

In preparation for competition, the training calendar is broken down into cycles, or phases, that sometimes extend over an entire year. The idea is to prepare for a "peak" performance at the time of competition. Different types of exercises at varying intensities, volume, frequency, and rest intervals are performed over a specific time period. Table 6.5 summarizes the characteristics of each cycle.

Although periodized training is commonly implemented prior to a competitive event, evidence to support the efficacy of periodization is limited.^102,170,192 Despite this, periodized training also has been used on a limited basis in the clinical setting for injured athletes during the advanced stage of rehabilitation.⁹⁶

Integration of Function

Balance of Stability and Active Mobility

Control of the body during functional movement and the ability to perform functional tasks require a balance of active movement superimposed on a stable background of neuromuscular control. Sufficient performance of agonist and antagonist muscles about a joint contributes to the dynamic stability of individual joints. For example, a person must be able to hold the trunk erect and stabilize the spine while lifting a heavy object. Stability is also necessary to control quick changes of direction during functional movements. Hence, a resistance exercise program must address the static strength as well as the dynamic strength of the trunk and extremities.

Balance of Strength, Power, and Endurance

Functional tasks related to daily living, occupational, and recreational activities require many combinations of muscle strength, power, and endurance. Various motor tasks require slow and controlled movements, rapid movements, repeated movements, and long-term positioning. Analysis of the tasks a patient would like to be able to do provides the framework for a task-specific resistance exercise program.

Task-Specific Movement Patterns with Resistance Exercise

There is a place in a resistance exercise program for strengthening isolated muscle groups as well as strengthening muscles in combined patterns. Applying resistance during exercise in anatomical planes, diagonal patterns, and combined task-specific movement patterns should be integral components of a carefully progressed resistance exercise program. Use of simulated functional movements under controlled, supervised conditions is a means to return a patient safely to independent functional activities.²⁰⁵

Pushing, pulling, lifting, and holding activities, for example, first can be done against a low level of resistance for a limited number of repetitions. Over time, a patient can gradually return to using the same movements during functional activities in an unsupervised work or home setting. The key to successful self-management is to teach a patient how to judge the speed, level, and duration of tension generation in muscle combined with the appropriate timing that is necessary to perform a motor task safely and efficiently.

The types of exercise selected for a resistance training program are contingent on many factors, including the cause and extent of primary and secondary impairments. Deficits in muscle performance, the stage of tissue healing, the condition of joints and their tolerance to compression and movement, the general abilities (physical and cognitive) of the patient, the availability of equipment, and of course, the patient's goals and the intended functional outcomes of the program must be considered. A therapist has an array of exercises from which to choose to design a resistance exercise program to meet the individual needs of each patient. There is no one best form or type of resistance training. Prior to selecting specific types of resistance exercise for a patient's rehabilitation program, a therapist may want to consider the questions listed in Box 6.6.

Application of the SAID principle based on the concept of specificity of training is key to making sound exercise decisions. In addition to selecting the appropriate types of exercise, a therapist must also make decisions about the intensity, volume, order, frequency, rest interval, and other factors discussed in the previous section of this chapter for effective progression of resistance training. Table 6.6 summarizes general guidelines for progression of exercise.

The types of exercise presented in this section are static (isometric) and dynamic, concentric and eccentric, isokinetic, and open-chain and closed-chain exercise, as well as manual and mechanical and constant and variable resistance exercises. The benefits, limitations, and applications of each of these forms of resistance exercise are analyzed and discussed. When available, supporting evidence from the scientific literature is summarized.

Manual and Mechanical Resistance Exercise

From a broad perspective, a load can be applied to a contracting muscle in two ways: manually or mechanically. The benefits and limitations of these two forms of resistance training are summarized in a later section of this chapter (see Boxes 6.14 and 6.15).

Manual Resistance Exercise

Manual resistance exercise is a type of active-resistive exercise in which resistance is provided by a therapist or other health professional. A patient can be taught how to apply self-resistance to selected muscle groups. Although the amount of resistance cannot be measured quantitatively, this technique is useful in the early stages of an exercise program when the muscle to be strengthened is weak and can overcome only minimal to moderate resistance. It is also useful when the range of joint movements needs to be carefully controlled. The amount of resistance given is limited only by the strength of the therapist.

NOTE: Techniques for application of manual resistance exercises in anatomical planes and diagonal patterns are presented in later sections of this chapter.

Mechanical Resistance Exercise

Mechanical resistance exercise is a form of active-resistive exercise in which resistance is applied through the use of equipment or mechanical apparatus. The amount of resistance can be measured quantitatively and incrementally progressed over time. It is also useful when the amount of resistance necessary is greater than what the therapist can apply manually.

NOTE: Systems and regimens of resistance training that involve the use of mechanical resistance, such as progressive resistive exercise (PRE), circuit weight training, and velocity spectrum rehabilitation, and the advantages and disadvantages of various types of resistance equipment are addressed later in this chapter.

Isometric Exercise (Static Exercise)

Isometric exercise is a static form of exercise in which a muscle contracts and produces force without an appreciable change in the length of the muscle and without visible joint motion.^182,210 Although there is no mechanical work done (force × distance), a measurable amount of tension and force output are produced by the muscle.Sources of resistance for isometric exercise include holding against a force applied manually, holding a weight in a particular position, maintaining a position against the resistance of body weight, or pushing or pulling an immovable object.

During the 1950s and 1960s, isometric resistance training became popular as an alternative to dynamic resistance exercise and initially was thought to be a more effective and efficient method of muscle strengthening. Isometric strength gains of 5% per week were reported when healthy subjects performed a single, near-maximal isometric contraction everyday over a 6-week period.¹³³ However, replications of this study called into question some of the original findings, particularly the rapid rate of strength gain.

Repetitive isometric contractions, for example a set of 20 per day, held for 6 seconds each against near-maximal resistance has since been shown to be a more effective method to improve isometric strength. A cross-exercise effect (a limited increase in strength of the contralateral, unexercised muscle group), as the result of transfer of training, also has been observed with maximum isometric training.⁶⁷

Rationale for Use of Isometric Exercise

The need for static strength and endurance is apparent in almost all aspects of control of the body during functional activities. Loss of static muscle strength occurs rapidly with immobilization and disuse, with estimates from 8% per week¹⁸⁷ to as much as 5% per day.²⁰⁸ these two aspects of static muscle performance, it has been suggested that muscular endurance plays a more important role than muscle strength in maintaining sufficient postural stability and in preventing injury during daily living tasks.¹⁹² For example, the postural muscles of the trunk and lower extremities must contract isometrically to hold the body erect against gravity and provide a background of stability for balance and functional movements in an upright position. Dynamic stability of joints is achieved by activating and maintaining a low level of co-contraction—that is, concurrent isometric contractions of antagonist muscles that surround joints.¹⁹⁵ The importance of isometric strength and endurance in the elbow, wrist, and finger musculature, for example, is apparent when a person holds and carries a heavy object for an extended period of time.

With these examples in mind, there can be no doubt that isometric exercises are an important part of a rehabilitation program designed to improve functional abilities. The rationale and indications for isometric exercise in rehabilitation are summarized in Box 6.7.

Types of Isometric Exercise

Several forms of isometric exercise with varying degrees of resistance and intensity of muscle contractions serve different purposes during successive phases of rehabilitation. All but one type (muscle setting) incorporate some form of significant resistance and, therefore, are used to improve static strength or develop sustained muscular control (endurance). Because no appreciable resistance is applied, muscle setting technically is not a form of resistance exercise but is included in this discussion to show a continuum of isometric exercise that can be used for multifaceted goals in a rehabilitation program.

Muscle-setting exercises. Setting exercises involve low-intensity isometric contractions performed against little to no resistance. They are used to decrease muscle pain and spasm and to promote relaxation and circulation after injury to soft tissues during the acute stage of healing. Two common examples of muscle setting are of the quadriceps and gluteal muscles.

Because muscle setting is performed against no appreciable resistance, it does not improve muscle strength except in very weak muscles. However, setting exercises can retard muscle atrophy and maintain mobility between muscle fibers when immobilization of a muscle is necessary to protect healing tissues during the very early phase of rehabilitation.

Stabilization exercises. This form of isometric exercise is used to develop a submaximal but sustained level of co-contraction to improve postural stability or dynamic stability of a joint by means of midrange isometric contractions against resistance in antigravity positions and in weight-bearing postures if weight bearing is permissible.¹⁹⁵ Body weight or manual resistance typically is the source of resistance.

Various terms are used to describe stabilization exercises. They include rhythmic stabilization and alternating isometrics, two techniques associated with proprioceptive neuromuscular facilitation (PNF) described later in the chapter.^220,288 Stabilization exercises that focus on trunk/postural control are referred to by a variety of descriptors including dynamic, core, and segmental stabilization exercises. Applications of these exercises are addressed in Chapter 16. Equipment, such as the BodyBlade^® (see Fig 6.50) and stability balls are designed for dynamic stabilization exercises.

Multiple-angle isometrics. This term refers to a system of isometric exercise in which resistance is applied, manually or mechanically, at multiple joint positions within the available ROM.⁵⁷ This approach is used when the goal of exercise is to improve strength throughout the ROM when joint motion is permissible but dynamic resistance exercise is painful or inadvisable.

Characteristics and Effects of Isometric Training

Effective use of isometric exercise in a resistance training program is founded on an understanding of its characteristics and its limitations.

Intensity of muscle contraction. The amount of tension that can be generated during an isometric muscle contraction depends in part on joint position and the length of the muscle at the time of contraction.²⁸⁵ It is sufficient to use an exercise intensity (load) of at least 60% of a muscle's maximum voluntary contraction (MVC) to improve strength.^162,285 The amount of resistance against which the muscle is able to hold varies and needs to be adjusted at different points in the range. Resistance must be progressively increased to continue to overload the muscle as it becomes stronger.

Duration of muscle activation. To achieve adaptive changes in static muscle performance, an isometric contraction should be held for 6 seconds and no more than 10 seconds because muscle fatigue develops rapidly. This allows sufficient time for peak tension to develop and for metabolic changes to occur in the muscle.^133,192 A 10-second contraction allows a 2-second rise time, a 6-second hold time, and a 2-second fall time.⁵⁷

Repetitive contractions. Use of repetitive contractions, held for 6 to 10 seconds each, decreases muscle cramping and increases the effectiveness of the isometric regimen.

Joint angle and mode specificity. Gains in muscle strength occur only at or closely adjacent to the training angle.^161,162,285 Physiological overflow is minimal, occurring no more than 10° in either direction from the training angle.¹⁶² Therefore, when performing multiple-angle isometrics, resistance at four to six points in the ROM typically is recommended. Additionally, isometric resistance training is mode-specific, causing increases in static strength with little to no impact on dynamic strength (concentric or eccentric).

Sources of resistance. It is possible to perform a variety of isometric exercises with or without equipment. For example, multiple-angle isometrics can be carried out against manual resistance or by simply having the patient push against an immovable object, such as a door frame or a wall.

Equipment designed for dynamic exercise can be adapted for isometric exercise. A weight-pulley system that provides resistance greater than the force-generating capacity of a muscle leads to a resisted isometric exercise. Most isokinetic devices can be set up with the velocity set at 0°/sec at multiple joint angles for isometric resistance at multiple points in the ROM.

PRECAUTION: Breath-holding commonly occurs during isometric exercise, particularly when performed against substantial resistance. This is likely to cause a pressor response as the result of the Valsalva maneuver, causing a rapid increase in blood pressure.⁹⁴ Rhythmic breathing, emphasizing exhalation during the contraction, should always be performed during isometric exercise to minimize this response.

CONTRAINDICATION: High-intensity isometric exercises may be contraindicated for patients with a history of cardiac or vascular disorders.

Dynamic Exercise: Concentric and Eccentric

A dynamic muscle contraction causes joint movement and excursion of a body segment as the muscle contracts and shortens (concentric muscle action) or lengthens under tension (eccentric muscle action). As represented in Figure 6.7, the term concentric exercise refers to a form of dynamic muscle loading in which tension in a muscle develops and physical shortening of the muscle occurs as an external force (resistance) is overcome, as when lifting a weight. In contrast, eccentric exercise involves dynamic loading of a muscle beyond its force-producing capacity, causing physical lengthening of the muscle as it attempts to control the load, as when lowering a weight.

During concentric and eccentric exercise, resistance can be applied in several ways: (1) constant resistance, such as body weight, a free weight, or a simple weight-pulley system; (2) a weight machine that provides variable resistance; or (3) an isokinetic device that controls the velocity of limb movement.

NOTE: Although the term isotonic (meaning equal tension) has been used frequently to describe a resisted, dynamic muscle contraction, application of this terminology is incorrect. In fact, when a body segment moves through its available range, the tension that the muscle is capable of generating varies through the range as the muscle shortens or lengthens. This is due to the changing length-tension relationship of the muscle and the changing torque of the load.^182,210,248 Therefore, in this textbook "isotonic" is not used to describe dynamic resistance exercise.

Rationale for Use of Concentric and Eccentric Exercise

Both concentric and eccentric exercises have distinct value in rehabilitation and conditioning programs. Concentric muscle contractions accelerate body segments, whereas eccentric contractions decelerate body segments (e.g., during sudden changes of direction or momentum). Eccentric contractions also act as a source of shock absorption during high-impact activities.^63,175

A combination of concentric and eccentric muscle action is evident in countless tasks of daily life, such as walking up and down inclines, ascending and descending stairs, rising from a chair and sitting back down, or picking up or setting down an object. Hence, it is advisable to incorporate a variety of concentric and eccentric resistance exercises in a rehabilitation progression for patients with impaired muscle performance to improve muscle strength, power, or endurance and to meet functional demands.

Special Considerations for Eccentric Training

Eccentric training, in particular, is considered an essential component of rehabilitation programs following musculoskeletal injury or surgery and in conditioning programs to reduce the risk of injury or reinjury associated with activities that involve high-intensity deceleration, quick changes of direction, or repetitive eccentric muscle contractions.^175,213,254 Eccentric training also is thought to improve sport-related physical performance.^10,175

Traditionally, regimens of exercise that emphasize high-intensity, eccentric loading, such as eccentric isokinetic training or plyometric training (see Chapter 23) have been initiated during the advanced phase of rehabilitation to prepare a patient for high-demand sports or work-related activities.¹⁷⁵ Recently, however, progressive eccentric training early in the rehabilitation process has been advocated to more effectively reduce deficits in strength and physical performance that often persist following a musculoskeletal injury or surgery. However, the safety of early implementation of eccentric resistance exercise must first be examined.

Characteristics and Effects of Concentric and Eccentric Exercise

A summary of the characteristics and effects of eccentric versus concentric resistance exercise is noted in Box 6.8.

Exercise load and strength gains. A maximum concentric contraction produces less force than a maximum eccentric contraction under the same conditions (see Fig. 6.6). In other words, greater loads can be lowered than lifted. This difference in the magnitude of loads that can be controlled by concentric versus eccentric muscle contractions may be associated with the contributions of the contractile and noncontractile components of muscle. When a load is lowered during an eccentric exercise, the force exerted by the load is controlled not only by the active, contractile components of muscle but also by the noncontractile connective tissue in and around the muscle. In contrast, when a weight is lifted during concentric exercise, only the contractile components of the muscle lift the load.⁶³

With a concentric contraction, greater numbers of motor units must be recruited to control the same load compared to an eccentric contraction, suggesting that concentric exercise has less mechanical efficiency than eccentric exercise.^63,78 Consequently, it requires more effort by a patient to control the same load during concentric exercise than during eccentric exercise. As a result, when a weight is lifted and lowered, maximum resistance during the concentric phase of an exercise does not provide a maximum load during the eccentric phase.

If a resistance exercise program involves maximum effort during eccentric and concentric exercise and if the exercise load is increased gradually, eccentric training increases eccentric strength over the duration of a program to a greater degree than concentric training increases concentric strength. This may occur because greater loads can be used for eccentric than concentric training.²³²

PRECAUTION: There is greater stress on the cardiovascular system (i.e., increased heart rate and arterial blood pressure) during eccentric exercise than during concentric exercise,⁶³ possibly because greater loads can be used for eccentric training. This underscores the need for rhythmic breathing during high-intensity exercise. (Refer to a later section of this chapter for additional information on cardiovascular precautions.)

Velocity of exercise. The velocity at which concentric or eccentric exercises are performed directly affects the force-generating capacity of the neuromuscular unit.^53,78 At slow velocities with a maximum load, an eccentric contraction generates greater tension than a concentric contraction. At slow velocities, therefore, a greater load (weight) can be lowered (with control) than lifted. As the velocity of exercise increases, concentric contraction tension rapidly and consistently decreases, whereas eccentric contraction forces increase slightly but then rapidly reach a plateau under maximum load conditions (see Fig. 6.6).

Energy expenditure. Against similar exercise loads, eccentric exercise is more efficient at a metabolic level than concentric exercise²³² —that is, eccentric muscle contractions consume less oxygen and energy stores than concentric contractions.⁴¹ Therefore, the use of eccentric activities such as downhill running may improve muscular endurance more efficiently than similar concentric activities because muscle fatigue occurs less quickly with eccentric exercise.^63,232

Specificity of training. Opinions and results of studies vary on whether the effects of training with concentric and eccentric contractions in the exercised muscle group are mode-specific. Although there is substantial evidence to support specificity of training,^{11,24,80,205,240,275} there is also some evidence to suggest that training in one mode leads to strength gains, though less significant, in the another mode.⁸⁵ For the most part, however, eccentric training is more mode-specific than concentric training.²³² Eccentric exercise also appears to be more velocity-specific than concentric exercise.²³² Therefore, because transfer of training is quite limited, selection of exercises that simulate the functional movements needed by a patient is always a prudent choice.

Cross-training effect. Both concentric²⁸³ and eccentric²⁸⁴ training have been shown to cause a cross-training effect—that is, a slight increase in strength occurs over time in the same muscle group of the opposite, unexercised extremity. This effect, sometimes referred to as cross-exercise, also occurs with high-intensity exercise that involves a combination of concentric and eccentric contractions (lifting and lowering a weight).

This effect in the unexercised muscle group may be caused by repeated contractions of the unexercised extremity in an attempt to stabilize the body during high-effort exercise. Although cross-training is an interesting phenomenon, there is no evidence to suggest that a cross-training effect has a positive impact on a patient's functional capabilities.

Exercise-induced muscle soreness. Repeated and rapidly progressed, high-intensity eccentric muscle contractions are associated with a significantly higher incidence and severity of delayed-onset muscle soreness (DOMS) than occurs with high-intensity concentric exercise.^{16,43,106,212} Why DOMS occurs more readily with eccentric exercise is speculative, possibly the result of greater damage to muscle and connective tissue when heavy loads are controlled and lowered.^16,43 It also has been suggested that the higher incidence of DOMS associated with unaccustomed, high-intensity eccentric exercise may adversely affect the training-induced gains in muscle strength.^63,80,106

It should be noted that there is at least limited evidence to suggest that if the intensity and volume of concentric and eccentric exercise are equal, there is no significant difference in the degree of DOMS after exercise.¹⁰⁰ Further, if the intensity and volume of eccentric exercise is progressed gradually, DOMS does not occur.¹¹¹

Dynamic Exercise: Constant and Variable Resistance

The most common system of resistance training used with dynamic exercise against constant or variable resistance is progressive resistance exercise (PRE). A later section of this chapter, which covers systems of training using mechanical resistance, addresses PRE.

Dynamic Exercise: Constant External Resistance

Dynamic exercise against constant external resistance (DCER) is a form of resistance training in which a limb moves through a ROM against a constant external load,¹⁶⁸ provided by free weights such as a handheld or cuff weight, torque arm units (Fig. 6.8 A), weight machines, or weight-pulley systems.

This terminology—DCER exercise—is used in lieu of the term "isotonic (equal tension)" exercise because although the load (weight) selected does not change, the torque imposed by the weight and the tension generated by the muscle both change throughout the range of movement.^182,248 If the imposed load is less than the torque generated by the muscle, the muscle contracts concentrically and accelerates the load; if the load exceeds the muscle's torque production, the muscle contracts eccentrically to decelerate the load (see Fig. 6.7)

DCER exercise has an inherent limitation. When lifting or lowering a constant load, the contracting muscle is challenged maximally at only one point in the ROM in which the maximum torque of the resistance matches the maximum torque output of the muscle. A therapist needs to be aware of the changing torque of the exercise and the changing length-tension relationship of the muscle and modify body position and resistance accordingly to match where in the range the maximum load needs to be applied (see Figs. 6.46 and 6.47). Despite this limitation, DCER exercise has been and continues to be a mainstay of rehabilitation and fitness programs for effective muscle loading and subsequent training-induced improvements in muscle performance.

Variable Resistance Exercise

Variable resistance exercise, a form of dynamic exercise, addresses the primary limitation of dynamic exercise against a constant external load (DCER exercises). Specially designed resistance equipment imposes varying levels of resistance to the contracting muscles to load the muscles more effectively at multiple points in the ROM. The resistance is altered throughout the range by means of a weight-cable system that moves over an asymmetrically shaped cam, by a lever arm system (Fig. 6.8 B), or by hydraulic or pneumatic mechanisms.²⁴⁹ How effectively this equipment varies the resistance to match muscle torque curves is questionable.

Dynamic exercise with elastic resistance products (bands and tubing) also can be thought of, in the broadest sense, as variable resistance exercise because of the inherent properties of the elastic material and its response to stretch.^146,152,243 (Refer to the final section of this chapter for additional information on exercise with elastic resistance devices.)

NOTE: When dynamic exercise is performed against manual resistance, a skilled therapist can vary the load applied to the contracting muscle throughout the ROM. The therapist adjusts the resistance based on the patient's response so the muscle is appropriately loaded at multiple portions of the ROM.

Special Considerations for DCER and Variable Resistance Exercise

Excursion of limb movement. During either DCER or variable resistance exercise, the excursion of limb movement is controlled exclusively by the patient (with the exception of exercising on resistance equipment that has a range-limiting device). When free weights, weight-pulley systems, and elastic devices are used, stabilizing muscles are recruited, in addition to the targeted muscle group, to control the arc and direction of limb movement.

Velocity of exercise. Although most daily living, occupational, and sport activities occur at medium to fast velocities of limb movement, exercises must be performed at a relatively slow-velocity to avoid momentum and uncontrolled movements, which could jeopardize the safety of the patient. (As a point of reference, dynamic exercise with a free weight typically is performed at about 60°/sec.⁵⁷) Consequently, the training-induced improvements in muscle strength that occur only at slow velocities may not prepare the patient for activities that require rapid bursts of strength or quick changes of direction.

Isokinetic Exercise

Isokinetic exercise is a form of dynamic exercise in which the velocity of muscle shortening or lengthening and the angular limb velocity is predetermined and held constant by a rate-limiting device known as an isokinetic dynamometer (Fig. 6.9).^{57,83,138,200} The term isokinetic refers to movement that occurs at an equal (constant) velocity. Unlike DCER exercise in which a specific weight (amount of resistance) is selected and superimposed on the contracting muscle, in isokinetic resistance training the velocity of limb movement—not the load—is manipulated. The force encountered by the muscle depends on the extent of force applied to the equipment.^5,138

Isokinetic exercise is also called accommodating resistance exercise.¹³⁸ Theoretically, if an individual is putting forth a maximum effort during each repetition of exercise, the contracting muscle produces variable but maximum force output, consistent with the muscle's variable tension-generating capabilities at all portions in the range of movement, not at only one small portion of the range as occurs with DCER training. Although early advocates of isokinetic training suggested it was superior to resistance training with free weights or weight-pulley systems, this claim has not been well supported by evidence. Today, use of isokinetic training is regarded as one of many tools that can be integrated into the later stages of rehabilitation.⁵

Characteristics of Isokinetic Training

A brief overview of the key characteristics of isokinetic exercise is addressed in this section. For more detailed information on isokinetic testing and training, a number of resources are available.^{5,57,81,83,122}

Constant velocity. Fundamental to the concept of isokinetic exercise is that the velocity of muscle shortening or lengthening is preset and controlled by the unit and remains constant throughout the ROM.

Range and selection of training velocities. Isokinetic training affords a wide range of exercise velocities in rehabilitation from very slow to fast velocities. Current dynamometers manipulate the velocity of limb movement from 0°/sec (isometric mode) up to 500°/sec. As shown in Table 6.7, these training velocities are classified as slow, medium, and fast. This range theoretically provides a mechanism by which a patient can prepare for the demands of functional activities that occur at a range of velocities of limb movement.

Selection of training velocities should be as specific as possible to the demands of the anticipated functional tasks. The faster training velocities appear to be similar to or approaching the velocities of limb movements inherent in some functional motor skills such as walking or lifting.^5,299 For example, the average angular velocity of the lower extremity during walking has been calculated at 230° to 240°/sec.^5,57,299 Notwithstanding, the velocity of limb movements during many functional activities far exceeds the fastest training velocities available.

The training velocities selected also may be based on the mode of exercise (concentric or eccentric) to be performed. As noted in Table 6.7, the range of training velocities advocated for concentric exercise is substantially greater than for eccentric training.^5,83,122

Reciprocal versus isolated muscle training. Use of reciprocal training of agonist and antagonist muscles emphasizing quick reversals of motion is possible on an isokinetic dynamometer. For example, the training parameter can be set so the patient performs concentric contraction of the quadriceps followed by concentric contraction of the hamstrings. An alternative approach is to target the same muscle in the concentric mode, followed by the eccentric mode, thus strengthening only one muscle group at a time.²⁹⁸ Both of these approaches have merit.

Specificity of training. Isokinetic training for the most part is velocity-specific,^24,122,149 with only limited evidence of significant overflow from one training velocity to another.^143,273 Evidence of mode-specificity (concentric vs. eccentric) with isokinetic exercise is less clear.^{12,83,121,205,240}

Because isokinetic exercise tends to be velocity-specific, patients typically exercise at several velocities (between 90° and 360°/sec) using a system of training known as velocity spectrum rehabilitation.^5,57,83 (This approach to isokinetic training is discussed later in this chapter.)

Compressive forces on joints. During concentric exercise, as force output decreases, the compressive forces across the moving joint are less at faster angular velocities than at slow velocities.^5,57,81,83

Accommodation to fatigue. Because the resistance encountered is directly proportional to the force applied to the resistance arm of the isokinetic unit, as the contracting muscle fatigues, a patient is still able to perform additional repetitions even though the force output of the muscle temporarily diminishes.

Accommodation to a painful arc. If a patient experiences transient pain at some portion of the arc of motion during exercise, isokinetic training accommodates for the painful arc. The patient simply pushes less vigorously against the resistance arm to move without pain through that portion of the range. If a patient needs to stop a resisted motion because of sudden onset of pain, the resistance is eliminated as soon as the patient stops pushing against the torque arm of the dynamometer.

Training Effects and Carryover to Function

Numerous studies have shown that isokinetic training is effective for improving one or more of the parameters of muscle performance (strength, power, and muscular endurance).^{12,24,83,143,193,205} In contrast, only a limited number of studies have investigated the relationship between isokinetic training and improvement in the performance of functional skills. Two such studies indicated that the use of high-velocity concentric and eccentric isokinetic training was associated with enhanced performance (increased velocity of a tennis serve and throwing a ball).^85,201

Limitations in carryover. Several factors inherent in the design of most types of isokinetic equipment may limit the extent to which isokinetic training carries over to improvements in functional performance. Although isokinetic training affords a spectrum of velocities for training, the velocity of limb movement during many daily living and sport-related activities far exceeds the maximum velocity settings available on isokinetic equipment. In addition, limb movements during most functional tasks occur at multiple velocities, not at a constant velocity, depending on the conditions of the task.

Furthermore, isokinetic exercise usually isolates a single muscle or opposing muscle groups, involves movement of a single joint, is uniplanar, and does not involve weight bearing. Although isolation of a single muscle can be beneficial in remediating strength deficits in specific muscle groups, most functional activities require contractions of multiple muscle groups and movement of multiple joints in several planes of motion, some in weight-bearing positions. It is important to note, however, that some of these limitations can be addressed by adapting the setup of the equipment to allow multi-axis movements in diagonal planes or multi-joint resisted movements with the addition of an attachment for closed-chain training.

Special Considerations for Isokinetic Training

Availability of Equipment

From a pragmatic perspective, one limitation of isokinetic exercise is that a patient can incorporate this form of exercise into a rehabilitation program only by going to a facility where the equipment is available. In addition, a patient must be given assistance to set up the equipment and often requires supervision during exercise. These considerations contribute to high costs for the patient enrolled in a long-term rehabilitation program.

Appropriate Setup

The setups recommended in the product manuals often must be altered to ensure that the exercise occurs in a position that is safe for a particular joint. For example, even though a manufacturer may describe a 90°/90° position of the shoulder and elbow for strengthening the shoulder rotators, exercising with the arm at the side may be a safer, more comfortable position.

Initiation and Progression of Isokinetic Training During Rehabilitation

Isokinetic training typically is begun in the later stages of rehabilitation, when active motion through the full (or partial) ROM is pain-free. Suggested guidelines for implementation and progression are summarized in Box 6.9.^5,57,83,122

Open-Chain and Closed-Chain Exercise

Background

In clinical practice and in the rehabilitation literature, functional activities and exercises commonly are categorized as having weight-bearing or nonweight-bearing characteristics. Another frequently used method of classifying movements and exercises is based on "open or closed kinetic chain" and "open or closed kinematic chain" concepts. These concepts, which were introduced during the 1950s and 1960s in the human biomechanics and kinesiology literature by Steindler²⁵⁶ and Brunnstrom,³² respectively, were proposed to describe how segments (structures) and motions of the body are linked and how muscle recruitment changes with different types of movement and in response to different loading conditions in the environment.

In his analysis of human motion, Steindler²⁵⁶ proposed that the term "open kinetic chain" applies to completely unrestricted movement in space of a peripheral segment of the body, as in waving the hand or swinging the leg. In contrast, he suggested that during closed kinetic chain movements the peripheral segment meets with "considerable external resistance." He stated that if the terminal segment remains fixed, the encountered resistance moves the proximal segments over the stationary distal segments. Steindler also noted that a closed kinetic chain motion in one joint is accompanied by motions of adjacent joints that occur in reasonably predictable patterns.

Both Steindler and Brunnstrom pointed out that the action of a muscle changes when the distal segment is free to move versus when it is fixed in place. For example, in an open chain the tibialis posterior muscle functions to invert and plantarflex the foot and ankle. In contrast, during the stance phase of gait (during loading), when the foot is planted on the ground, the tibialis posterior contracts to decelerate pronation of the subtalar joint and supinate the foot to externally rotate the lower leg during mid and terminal stance.

During the late 1980s and early 1990s, clinicians and researchers in rehabilitation, who were becoming familiar with the open and closed kinetic (or kinematic) chain approach to classifying human motion, began to describe exercises based on these concepts.^221,230,274

Controversy and Inconsistency in Use of Open-Chain and Closed-Chain Terminology

Although use of open- and closed-chain terminology to describe exercises has become prevalent in clinical practice and in the rehabilitation literature, a lack of consensus has emerged about how—or even if—this terminology should be used and what constitutes an open-chain versus a closed-chain exercise.^{25,71,72,124,251,291}

One source of inconsistency is whether weight bearing is an inherent component of closed kinetic chain motions. Steindler²⁵⁶ did not specify that weight bearing must occur for a motion to be categorized as closed kinetic chain, but many of his examples of closed-chain movements, particularly in the lower extremities, involved weight bearing. In the rehabilitation literature, descriptions of a closed kinetic chain often do^61,101—but sometimes do not^127,251—include weight bearing as a necessary element. One resource suggested that all weight-bearing exercises involve some elements of closed-chain motions, but not all closed-chain exercises are performed in weight-bearing positions.²⁵¹

Another point of ambiguity is whether the distal segment must be absolutely fixed in place to a surface and not moving on a surface to be classified as a closed-chain motion. Steindler²⁵⁶ described this as one of the conditions of a closed kinetic chain motion. However, another condition in his description of closed-chain motions is that if the "considerable external resistance" is overcome, it results in movement of the peripheral segment. Examples Steindler cited were pushing a cart away from the body and lifting a load. Consequently, some investigators have identified a bench press exercise, a seated or reclining leg press exercise, or cycling as closed-chain exercises because they involve pushing motions and axial loading.^25,60 If these exercises fit under the closed-chain umbrella, does an exercise in which the distal segment slides across the support surface also qualify as a closed-chain motion? Opinion is divided.

Lifting a handheld weight or pushing against the force arm of an isokinetic dynamometer are consistently cited in the literature as examples of open-chain exercises.^* Although there is no axial loading in these exercises, considering Steindler's condition for closed-chain motion just discussed, should these exercises more correctly be classified as closed-chain rather than open-chain exercises in that the distal segment is overcoming considerable external resistance? Again, there continues to be no consensus.

Given the complexity of human movement, it is not surprising that a single classification system cannot adequately group the multitude of movements found in functional activities and therapeutic exercise interventions.

Alternatives to Open-Chain and Closed-Chain Terminology

To address the unresolved issues associated with open-chain and closed-chain terminology, several authors have offered alternative or additional terms to classify exercises. One suggestion is to use the terms, "distally fixated" and "nondistally fixated" in lieu of closed-chain and open-chain.²⁰⁴ Another suggestion is to add a category dubbed partial kinetic chain²⁹¹ to describe exercises in which the distal segment (hand or foot) meets resistance but is not absolutely stationary, such as using a leg press machine, stepping machine, or slide board. The term closed kinetic chain is then reserved for instances when the terminal segment does not move.

To avoid use of the open- or closed-chain terminology, another classification system categorizes exercises as either joint isolation exercises (movement of only one joint segment) or kinetic chain exercises (simultaneous movement of multiple segments that are linked).^159,221 Boundaries of movement of the peripheral segment (movable or stationary) or loading conditions (weight-bearing or nonweight-bearing) are not parameters of this terminology. However, other, more complex classification systems do take these conditions into account.^72,181

An additional option is to describe the specific conditions of exercises. Using this approach, most open-chain exercises could be described as single-joint weight-bearing exercises, and most closed-chain exercises would be identified as multiple-joint weight-bearing exercises.¹³¹

Despite the suggested alternative terminology, open- and closed-chain terminology continues to be widely used in practice settings and in the literature.^† Therefore, recognizing the many inconsistencies and shortcomings of the kinetic or kinematic chain terminology and that many exercises and functional activities involve a combination of open- and closed-chain motions, the authors of this textbook have elected to continue to use open- and closed-chain terminology to describe exercises.

Characteristics of Open-Chain and Closed-Chain Exercises

The following operational definitions and characteristics of open- and closed-chain exercises are presented for clarity and as the basis for the discussion of open- and closed-chain exercises described throughout this textbook. The parameters of the definitions are those most frequently noted in the current literature. Common characteristics of open- and closed-chain exercises are compared in Table 6.8.

Open-Chain Exercises

Open-chain exercises involve motions in which the distal segment (hand or foot) is free to move in space, without necessarily causing simultaneous motions at adjacent joints.^{60,84,86,101,159} Limb movement only occurs distal to the moving joint, and muscle activation occurs in the muscles that cross the moving joint. For example, during knee flexion in an open-chain exercise (Fig. 6.10), the action of the hamstrings is independent of recruitment of other hip or ankle musculature. Open-chain exercises also are typically performed in nonweight-bearing positions.^{61,101,197,301} In addition, during resistance training, the exercise load (resistance) is applied to the moving distal segment.

Closed-Chain Exercises

Closed-chain exercises involve motions in which the body moves on a distal segment that is fixed or stabilized on a support surface. Movement at one joint causes simultaneous motions at distal as well as proximal joints in a relatively predictable manner. For example, when performing a bilateral short-arc squatting motion (mini-squat) (Fig. 6.11) and then returning to an erect position, as the knees flex and extend, the hips and ankles move in predictable patterns.

Closed-chain exercises typically are performed in weight-bearing positions.^{60,85,101,127,197,301} Examples in the upper extremities include balance activities in quadruped, press-ups from a chair, wall push-offs, or prone push-ups; examples in the lower extremities include lunges, squats, step-up or step-down exercises, or heel rises to name a few.

NOTE: In this textbook, as in some other publications,^{84,86,127,262} inclusive in the scope of closed-chain exercises are weight-bearing activities in which the distal segment moves but remains in contact with the support surface, as when using a bicycle, cross-country ski machine, or stair-stepping machine. In the upper extremities a few nonweight-bearing activities qualify as closed-chain exercises, such as pull-ups on a trapeze in bed or chin-ups at an overhead bar.

Rationale for Use of Open-Chain and Closed-Chain Exercises

The rationale for selecting open- or closed-chain exercises is based on the goals of an individualized rehabilitation program and a critical analysis of the potential benefits and limitations inherent in either form of exercise. Because functional activities involve many combinations and considerable variations of open- and closed-chain motions, inclusion and integration of task-specific open-chain and closed-chain exercises into a rehabilitation or conditioning program is both appropriate and prudent.

A summary of the benefits and limitations of open- and closed-chain exercises and the rationale for their use follows. Whenever possible, presumed benefits and limitations or comparisons of both forms of exercise are analyzed in light of existing scientific evidence. Some of the reported benefits and limitations are supported by evidence, whereas others are often founded on opinion or anecdotal reports. Evidence is presented as available.

NOTE: Most reports and investigations comparing or analyzing open- or closed-chain exercises have focused on the knee, in particular the ACL or patellofemoral joint. Far fewer articles have addressed the application or impact of open- and closed-chain exercises on the upper extremities.

Isolation of Muscle Groups

Open-chain testing and training identifies strength deficits and improves muscle performance of individual muscles or muscle groups more effectively than closed-chain exercises. The possible occurrence of substitute motions that compensate for and mask strength deficits of individual muscles is greater with closed-chain exercise than open-chain exercise.

Control of Movements

During open-chain resisted exercises a greater level of control is possible with a single moving joint than with multiple moving joints as occurs during closed-chain training. With open-chain exercises, stabilization is usually applied externally by a therapist's manual contacts or with belts or straps. In contrast, during closed-chain exercises the patient most often uses muscular stabilization to control joints or structures proximal and distal to the targeted joint. The greater levels of control afforded by open-chain training are particularly advantageous during the early phases of rehabilitation.

Joint Approximation

Almost all muscle contractions have a compressive component that approximates the joint surfaces and provides stability to the joint whether in open- or closed-chain situations.^182,210,248 Joint approximation also occurs during weight bearing and is associated with lower levels of shear forces at a moving joint. This has been demonstrated at the knee (decreased anterior or posterior tibiofemoral translation)^300,301 and possibly at the glenohumeral joint.²⁸¹ The joint approximation that occurs with the axial loading and weight bearing during closed-chain exercises is thought to cause an increase in joint congruency, which in turn contributes to stability.^60,84

Co-activation and Dynamic Stabilization

Because most closed-chain exercises are performed in weight-bearing positions, it has been assumed and commonly reported in the neurorehabilitation literature that closed-chain exercises stimulate joint and muscle mechanoreceptors, facilitate co-activation of agonists and antagonists (co-contraction), and consequently promote dynamic stability.^220,264,279 During a standing squat, for example, the quadriceps and hamstrings are thought to contract concurrently to control the knee and hip, respectively. In studies of lower extremity closed-chain exercises and activity of the knee musculature, this assumption has been supported^30,50,292 and refuted.⁸⁷

In the upper extremity, closed-chain exercises in weight-bearing positions are also thought to cause co-activation of the scapular and glenohumeral stabilizers and, therefore, to improve dynamic stability of the shoulder complex.^84,291 The assumption seems plausible, but evidence of co-contraction of muscles of the shoulder girdle during weight-bearing exercises, such as a prone push-up or a press-up from a chair, is limited,¹⁷⁶ making it difficult for clinicians to draw conclusions or make evidence-based decisions.

There is also some thought that co-activation (co-contraction) of agonist and antagonist muscle groups may occur with selected open-chain exercises. Exercise interventions—such as alternating isometrics associated with PNF,^220,264,279 some stretch-shortening drills performed in nonweight-bearing positions,²⁹³ use of a BodyBlade^® (see Fig. 6.50), and high-velocity isokinetic training—may stimulate co-activation of muscle groups to promote dynamic stability. However, evidence of this possibility is limited.

In some studies of open-chain, high-velocity, concentric isokinetic training of knee musculature,^77,123 co-activation of agonist and antagonist muscle groups was noted briefly at the end the range of knee extension. Investigators speculated that the knee flexors fired and contracted eccentrically at the end of the range of knee extension to decelerate the limb just before contact was made with the ROM stop. However in another study, there was no evidence of co-activation of knee musculature or decreased anterior tibial translation with maximum effort, slow-velocity (60°/sec), open-chain training.¹⁷³

PRECAUTION: High-load, open-chain exercise may have an adverse effect on unstable, injured, or recently repaired joints, as demonstrated in the ACL-deficient knee.^{87,150,292,300}

Proprioception, Kinesthesia, Neuromuscular Control, and Balance

Conscious awareness of joint position or movement is one of the foundations of motor learning during the early phase of training for neuromuscular control of functional movements. After soft tissue or joint injury, proprioception and kines-thesia are disrupted and alter neuromuscular control. Reestablishing the effective, efficient use of sensory information to initiate and control movement is a high priority in rehabilitation.¹⁸⁰ Studies of the ACL-reconstructed knee have shown that proprioception and kinesthesia do improve after rehabilitation.^19,178

It is thought that closed-chain training provides greater proprioceptive and kinesthetic feedback than open-chain training. Theoretically, because multiple muscle groups that cross multiple joints are activated during closed-chain exercise, more sensory receptors in more muscles and intra-articular and extra-articular structures are activated to control motion than during open-chain exercises. The weight-bearing element (axial loading) of closed-chain exercises, which causes joint approximation, is believed to stimulate mechanoreceptors in muscles and in and around joints to enhance sensory input for the control of movement.^*

Lastly, closed-chain positioning is the obvious choice to improve balance and postural control in the upright position. Balance training is believed to be an essential element of the comprehensive rehabilitation of patients after musculoskeletal injuries or surgery, particularly in the lower extremities, to restore functional abilities and reduce the risk of re-injury.¹⁵⁵ Activities and parameters to challenge the body's balance mechanisms are discussed in Chapter 8.

Carryover to Function and Injury Prevention

As already noted, there is ample evidence to demonstrate that both open- and closed-chain exercises effectively improve muscle strength, power, and endurance.^58,60,61 Evidence also suggests that if there is a comparable level of loading (amount of resistance) applied to a muscle group, EMG activity is similar regardless of whether open-chain or closed-chain exercises are performed.^25,72

That being said, and consistent with the principles of motor learning and task-specific training, exercises should be incorporated into a rehabilitation program that simulate the desired functions if the selected exercises are to have the most positive impact on functional outcomes.^{60,124,251,291}

Implementation and Progression of Open-Chain and Closed-Chain Exercises

Principles and general guidelines for the implementation and progression of open-chain and closed-chain exercises are similar with respect to variables such as intensity, volume, frequency, and rest intervals. These variables were discussed earlier in the chapter. Relevant features of closed-chain exercises and guidelines for progression are summarized in Table 6.9.

Introduction of Open-Chain Training

Because open-chain training typically is performed in nonweight-bearing postures, it may be the only option when weight bearing is contraindicated or must be significantly restricted or when unloading in a closed-chain position is not possible. Soft tissue pain and swelling or restricted motion of any segment of the chain may also necessitate the use of open-chain exercises at adjacent joints. After a fracture of the tibia, for example, the lower extremity usually is immobilized in a long leg cast, and weight bearing is restricted for at least a few weeks. During this period, hip strengthening exercises in an open-chain manner can still be initiated and gradually progressed until partial weight bearing and closed-chain activities are permissible.

Any activity that involves open-chain motions can be easily replicated with open-chain exercises, first by developing isolated control and strength of the weak musculature and then by combining motions to simulate functional patterns.

Closed-Chain Exercises and Weight-Bearing Restrictions: Use of Unloading

If weight bearing must be restricted, a safe alternative to open-chain exercises may be to perform closed-chain exercises while partial weight bearing on the involved extremity. This is simple to achieve in the upper extremity; but in the lower extremity, because the patient is in an upright position during closed-chain exercises, axial loading in one or both lower extremities must be reduced.

Use of aquatic exercises, as described in Chapter 9, or decreasing the percentage of body weight borne on the involved lower extremity in parallel bars are both feasible unloading strategies even though each has limitations. It is difficult to control the extent of weight bearing when performing closed-chain exercises in parallel bars. In addition, lower limb movements while standing in the parallel bars or in water tend to be slower than what typically occurs during functional tasks. An alternative is the use of a harnessing system to unload the lower extremities.¹⁵⁷ This system enables a patient to perform a variety of closed-chain exercises and to begin ambulation on a treadmill at functional speeds early in rehabilitation.

Progression of Closed-Chain Exercises

The parameters and suggestions for progression of closed-chain activities noted in Table 6.9 are not all-inclusive and are flexible. As a rehabilitation program progresses, more advanced forms of closed-chain training, such as plyometric training and agility drills (discussed in Chapter 23), can be introduced.^62,86 The selection and progression of activities should always be based on the discretion of the therapist and the patient's functional needs and response to exercise interventions.

General Principles of Resistance Training

The principles of resistance training presented in this section apply to the use of both manual and mechanical resistance exercises for persons of all ages, but these principles are not "set in stone." There are many instances when they may or should be modified based on the judgment of the therapist. Additional guidelines specific to the application of manual resistance exercise, PNF, and mechanical resistance exercise are addressed in later sections of this chapter.

Examination and Evaluation

As with all forms of therapeutic exercise, a comprehensive examination and evaluation is the cornerstone of an individualized resistance training program. Therefore, prior to initiating any form of resistance exercise:

• Perform a thorough examination of the patient, including a health history, systems review, and selected tests and measurements.

  • Determine qualitative and quantitative baselines of strength, muscular endurance, ROM, and overall level of functional performance against which progress can be measured.

• Interpret the findings to determine if the use of resistance exercise is appropriate or inappropriate at this time. Some questions that may need to be answered are noted in Box 6.10. Be sure to identify the most functionally relevant impairments, the goals the patient is seeking to achieve, and the expected functional outcomes of the exercise program.

• Establish how resistance training will be integrated into the plan of care with other therapeutic exercise interventions, such as stretching, joint mobilization techniques, balance training, and cardiopulmonary conditioning exercises.

• Re-evaluate periodically to document progress and determine if and how the dosage of exercises (intensity, volume, frequency, rest) and the types of resistance exercise should be adjusted to continue to challenge the patient.

Preparation for Resistance Exercises

• Select and prescribe the forms of resistance exercise that are appropriate and expected to be effective, such as whether to implement manual or mechanical resistance exercises, or both.

• If implementing mechanical resistance exercise, determine what equipment is needed and available.

• Review the anticipated goals and expected functional outcomes with the patient.

• Explain the exercise plan and procedures. Be sure that the patient and/or family understands and gives consent.

• Have the patient wear nonrestrictive clothing and supportive shoes appropriate for exercise.

• If possible, select a firm but comfortable support surface for exercise.

• Demonstrate each exercise and the desired movement pattern.

Implementation of Resistance Exercises

NOTE: These general guidelines apply to the use of dynamic exercises against manual or mechanical resistance. In addition to these guidelines, refer to special considerations and guidelines unique to the application of manual and mechanical resistance exercises in the sections of this chapter that follow.

Warm-Up

Prior to initiating resistance exercises, warm-up with light, repetitive, dynamic, site-specific movements without applying resistance. For example, prior to lower extremity resistance exercises, have the patient walk on a treadmill, if possible, for 5 to 10 minutes followed by flexibility exercises for the trunk and lower extremities.

Placement of Resistance

• Resistance typically is applied to the distal end of the segment in which the muscle to be strengthened attaches. Distal placement of resistance generates the greatest amount of external torque with the least amount of manual or mechanical resistance (load). For example, to strengthen the anterior deltoid, resistance is applied to the distal humerus as the patient flexes the shoulder (Fig. 6.12).

• Resistance may be applied across an intermediate joint if that joint is stable and pain-free and if there is adequate muscle strength supporting the joint. For example, to strengthen the anterior deltoid using mechanical resistance, a handheld weight is a common source of resistance.

• Revise the placement of resistance if pressure from the load is uncomfortable for the patient.

Direction of Resistance

During concentric exercise resistance is applied in the direction directly opposite to the desired motion, whereas during eccentric exercise resistance is applied in the same direction as the desired motion (see Fig. 6.12).

Stabilization

Stabilization is necessary to avoid unwanted, substitute motions.

• For nonweight-bearing resisted exercises, external stabilization of a segment usually is applied at the proximal attachment of the muscle to be strengthened. In the case of the biceps brachii muscle, for example, stabilization should occur at the anterior shoulder as elbow flexion is resisted (Fig. 6.13). Equipment such as belts or straps are effective sources of external stabilization.

• During multijoint resisted exercises in weight-bearing postures, the patient must use muscle control (internal stabilization) to hold nonmoving segments in proper alignment.

Intensity of Exercise/Amount of Resistance

NOTE: The intensity of the exercise (submaximal to near-maximal) must be consistent with the intended goals of resistance training and the type of muscle contraction as well as other aspects of dosage.

• Initially, have the patient practice the movement pattern against a minimal load to learn the correct exercise technique.

• Have the patient exert a forceful but controlled and pain-free effort. The level of resistance should be such that movements are smooth and nonballistic or tremulous.

• Adjust the alignment, stabilization, or the amount of resistance if the patient is unable to complete the available ROM, muscular tremor develops, or substitute motions occur.

Number of Repetitions, Sets, and Rest Intervals

• In general, for most adults, use 8 to 12 repetitions of a specific motion against a moderate exercise load. This typically induces expected acute and chronic responses—that is, muscular fatigue and adaptive gains in muscular strength, respectively.

• Decrease the amount of resistance if the patient cannot complete 8 to 12 repetitions.

• After a brief rest, perform additional repetitions—a second set of 8 to 12 repetitions, if possible.

• For progressive overloading, initially increase the number of repetitions or sets; at a later point in the exercise program, gradually increase the resistance.

Verbal or Written Instructions

When teaching an exercise using mechanical resistance or when applying manual resistance, use simple instructions that are easily understood. Do not use medical terminology or jargon. For example, tell the patient to "Bend and straighten your elbow" rather than "Flex and extend your elbow." Be sure that descriptions of resistance exercises to be performed in a home program are written and clearly illustrated.

Monitoring the Patient

Assess the patient's responses before, during, and after exercise. It may be advisable to monitor the patient's vital signs. Adhere to relevant precautions discussed in the next section of the chapter.

Cool-Down

Cool-down after a series of resistance exercises with rhythmic, unresisted movements, such as arm swinging, walking, or stationary cycling. Gentle stretching is also appropriate after resistance exercise.

Precautions for Resistance Exercise

Regardless of the goals of a resistance exercise program and the types of exercises prescribed and implemented, the exercises must not only be effective but safe. The therapist's interpretation of the examination's findings determines the exercise prescription. Awareness of precautions maximizes patient safety. General precautions for resistance training are summarized in Box 6.11.

Additional information about several of these precautions is presented in this section. Special considerations and precautions for children and older adults who participate in weight-training programs are addressed later in the chapter.

Valsalva Maneuver

The Valsalva maneuver (phenomenon), which is defined as an expiratory effort against a closed glottis, must be avoided during resistance exercise. The Valsalva maneuver is characterized by the following sequence. A deep inspiration is followed by closure of the glottis and contraction of the abdominal muscles. This increases intra-abdominal and intrathoracic pressures, which in turn forces blood from the heart, causing an abrupt, temporary increase in arterial blood pressure.¹⁵¹

During exercise the Valsalva phenomenon occurs most often with high-effort isometric⁹⁴ and dynamic¹⁸⁶ muscle contractions. It has been shown that the rise in blood pressure induced by an isometric muscle contraction is proportional to the percentage of maximum voluntary force exerted.¹⁸⁶ During isokinetic (concentric) testing, if a patient exerts maximum effort at increasing velocities, the rise in blood pressure appears to be the same at all velocities of movement despite the fact that the force output of the muscle decreases.⁷⁶ Although occurrence of the Valsalva phenomenon more often is thought to be associated with isometric^94,151 and eccentric⁶³ resistance exercise, a recent study¹⁸⁶ indicated that the rise in blood pressure appears to be based more on extent of effort—not strictly on the type (mode) of muscle contraction.

At-Risk Patients

The risk of complications from a rapid rise in blood pressure is particularly high in patients with a history of coronary artery disease, myocardial infarction, cerebrovascular disorders, or hypertension. Also at risk are patients who have undergone neurosurgery or eye surgery or who have inter-vertebral disk pathology. High-risk patients must be monitored closely.

Prevention During Resistance Exercise

• Caution the patient about breath-holding.

• Ask the patient to breathe rhythmically, count, or talk during exercise.

• Have the patient exhale when lifting and inhale when lowering an exercise load.⁸

• Be certain that high-risk patients avoid high-intensity resistance exercises.

Substitute Motions

If too much resistance is applied to a contracting muscle during exercise, substitute motions can occur. When muscles are weak because of fatigue, paralysis, or pain, a patient may attempt to carry out the desired movements that the weak muscles normally perform by any means possible.¹⁵⁸ For example, if the deltoid or supraspinatus muscles are weak or abduction of the arm is painful, a patient elevates the scapula (shrugs the shoulder) and laterally flexes the trunk to the opposite side to elevate the arm. It may appear that the patient is abducting the arm, but in fact that is not the case. To avoid substitute motions during exercise, an appropriate amount of resistance must be applied, and correct stabilization must be used with manual contacts, equipment, or by means of muscular (internal) stabilization by the patient.

Overtraining and Overwork

Exercise programs in which heavy resistance is applied or exhaustive training is performed repeatedly must be progressed cautiously to avoid a problem known as overtraining or overwork. These terms refer to deterioration in muscle performance and physical capabilities (either temporary or permanent) that can occur in healthy individuals or in patients with certain neuromuscular disorders.

In most instances, the uncomfortable sensation associated with acute muscle fatigue induces an individual to cease exercising. This is not necessarily the case in highly motivated athletes who are said to be overreaching in their training program¹⁰⁸ or in patients who may not adequately sense fatigue because of impaired sensation associated with a neuromuscular disorder.²¹⁹

Overtraining

The term overtraining is commonly used to describe a decline in physical performance in healthy individuals participating in high-intensity, high-volume strength and endurance training programs.^108,172 The terms chronic fatigue, staleness, and burnout are also used to describe this phenomenon. When overtraining occurs, the individual progressively fatigues more quickly and requires more time to recover from strenuous exercise because of physiological and psychological factors.

Overtraining is brought on by inadequate rest intervals between exercise sessions, too rapid progression of exercises, and inadequate diet and fluid intake. Fortunately, in healthy individuals, overtraining is a preventable, reversible phenomenon that can be resolved by tapering the training program for a period of time by periodically decreasing the volume and frequency of exercise (periodization).^{108,167,170,172}

Overwork

The term overwork, sometimes called overwork weakness, refers to progressive deterioration of strength in muscles already weakened by nonprogressive neuromuscular disease.²¹⁹ This phenomenon was first observed more than 50 years ago in patients recovering from polio who were actively involved in rehabilitation.²¹ In many instances the decrement in strength that was noted was permanent or prolonged. More recently, overwork weakness has been reported in patients with other nonprogressive neuromuscular diseases, such as Guillain-Barré syndrome.⁵⁶ Postpolio syndrome is also thought to be related to long-term overuse of weak muscles.⁹⁸

Overwork weakness has been produced in laboratory animals,¹²⁹ which provides some insight as to its cause. When strenuous exercise was initiated soon after a peripheral nerve lesion, the return of functional motor strength was retarded. It was suggested that this could be caused by excessive protein breakdown in the denervated muscle.

Prevention is the key to dealing with overwork weakness. Patients in resistance exercise programs who have impaired neuromuscular function or a systemic, metabolic, or inflammatory disease that increases susceptibility to muscle fatigue must be monitored closely, progressed slowly and cautiously, and re-evaluated frequently to determine their response to resistance training. These patients should not exercise to exhaustion and should be given longer and more frequent rest intervals during and between exercise sessions.^4,56

Exercise-Induced Muscle Soreness

Almost every individual, unaccustomed to exercise who begins a resistance training program, particularly a program that includes eccentric exercise, experiences muscle soreness. Exercise-induced muscle soreness falls into two categories: acute and delayed onset.

Acute Muscle Soreness

Acute muscle soreness develops during or directly after strenuous exercise performed to the point of muscle exhaustion.⁴⁴ This response occurs as a muscle becomes fatigued during acute exercise because of the lack of adequate blood flow and oxygen (ischemia) and a temporary buildup of metabolites, such as lactic acid and potassium, in the exercised muscle.^9,44 The sensation is characterized as a feeling of burning or aching in the muscle. It is thought that the noxious metabolic waste products may stimulate free nerve endings and cause pain. The muscle pain experienced during intense exercise is transient and subsides quickly after exercise when adequate blood flow and oxygen are restored to the muscle. An appropriate cool-down period of low-intensity exercise (active recovery) can facilitate this process.⁵¹

Delayed-Onset Muscle Soreness

After vigorous and unaccustomed resistance training or any form of muscular overexertion, DOMS, which is noticeable in the muscle belly or at the myotendinous junction,^70,104,144 begins to develop approximately 12 to 24 hours after the cessation of exercise. As was already pointed out in the discussion of concentric and eccentric exercise in this chapter, high-intensity eccentric muscle contractions consistently cause the most severe DOMS symptoms.^{16,63,78,100,104,215} Box 6.12 lists the signs and symptoms over the time course of DOMS. Although the time course varies, the signs and symptoms, which can last up to 10 to 14 days, gradually dissipate.^16,78,100

Etiology of DOMS. Despite years of research dating back to the early 1900s,¹⁴² the underlying mechanisms (mechanical, neural, or/or cellular) of tissue damage associated with DOMS is still unclear.^43,196 Several theories have been proposed, and some subsequently have been refuted. Early investigators proposed the metabolic waste accumulation theory, which suggested that both acute and delayed-onset muscle soreness was caused by a buildup of lactic acid in muscle after exercise. Although this is a source of muscle pain with acute exercise, this theory has been disproved as a cause of DOMS.²⁸⁰ Multiple studies have shown that it requires only about 1 hour of recovery after exercise to exhaustion to remove almost all lactic acid from skeletal muscle and blood.¹⁰⁴

The muscle spasm theory also was proposed as the cause of DOMS, suggesting that a feedback cycle of pain caused by ischemia and a buildup of metabolic waste products during exercise led to muscle spasm.⁶⁹ This buildup, it was hypothesized, caused the DOMS sensation and an ongoing reflex pain-spasm cycle that lasted for several days after exercise. The muscle spasm theory has been discounted in subsequent research that showed no increase in EMG activity and, therefore, no evidence of spasm in muscles with delayed soreness.²

Although studies on the specific etiology of DOMS continue, current research seems to suggest that DOMS is linked to some form of contraction-induced, mechanical disruption (microtrauma) of muscle fibers and/or connective tissue in and around muscle that results in degeneration of the tissue.^43,106 Evidence of tissue damage such as elevated blood serum levels of creatine kinase, is present for several days after exercise and is accompanied by inflammation and edema.^2,105,106

The temporary loss of strength and the perception of soreness or aching associated with DOMS appear to occur independently and follow different time courses. Strength deficits develop prior to the onset of soreness and persist after soreness has remitted.^66,212 Thus, force production deficits appear to be the result of muscle damage, possibly myofibrillar damage at the Z bands,^43,211 which directly affects the structural integrity of the contractile units of muscle, not neuromuscular inhibition as the result of pain.^211,212

Prevention and treatment of DOMS. Prevention and treatment of DOMS at the initiation of an exercise program after a short or long period of inactivity have been either ineffective or, at best, marginally successful. It is a commonly held opinion in clinical and fitness settings that the initial onset of DOMS can be prevented or at least kept to a minimum by progressing the intensity and volume of exercise gradually,^48,78 by performing low-intensity warm-up and cool-down activities,^68,78,247 or by gently stretching the exercised muscles before and after strenuous exercise.^68,247 Although these techniques are regularly advocated and employed, little to no evidence in the literature supports their efficacy in the prevention of DOMS.

There is some evidence to suggest that the use of repetitive concentric exercise prior to DOMS-inducing eccentric exercise does not entirely prevent but does reduce the severity of muscle soreness and other markers of muscle damage.²¹⁴ Paradoxically, a regular routine of resistance exercise, particularly eccentric exercise, prior to the onset of DOMS or after an initial episode of DOMS has developed and remitted.^9,43,44,48 This response is often referred to as the "repeated-bout effect," whereby a bout of eccentric exercise protects the muscle from damage from subsequent bouts of eccentric exercise.¹⁹⁶ It may well be that with repeated bouts of the same level of eccentric exercise or activity that caused the initial episode of DOMS, the muscle adapts to the physical stress, resulting in the prevention of additional episodes of DOMS.^9,43,175,196

Effective treatment of DOMS, once it has occurred, is continually being sought because, to date, the efficacy of DOMS treatment has been mixed. Evidence shows that continuation of a training program that has induced DOMS does not worsen the muscle damage or slow the recovery process.^43,215 Light, high-speed (isokinetic), concentric exercise has been reported to reduce muscle soreness and hasten the remediation of strength deficits associated with DOMS,¹²⁵ but other reports suggest no significant improvement in strength or relief of muscle soreness with light exercise.^74,282

The value of therapeutic modalities and massage techniques also is questionable. Electrical stimulation to reduce soreness has been reported to be effective^66,153 and ineffective.²⁸² Although cryotherapy (cold water immersion) after vigorous eccentric exercise reduces signs of muscle damage (creatine kinase activity), it has been reported to have little to no effect on the perpetuation of muscle tenderness or strength deficit.⁸⁸ Also, there is no significant evidence that postexercise massage, despite its widespread use in sports settings, reduces the signs and symptoms of DOMS.^153,272,282 Other treatments, such as hyperbaric oxygen therapy and nutritional supplements (vitamins C and E) also have been shown to have limited benefits.⁴⁸ However, use of compression sleeves^166,171 and topical salicylate creams, which provide an analgesic effect, may also reduce the severity of and hasten the recovery from DOMS-related symptoms.

In summary, although some interventions for the treatment of DOMS appear to have potential, a definitive treatment has yet to be determined.

Pathological Fracture

When an individual with known (or at high risk for) osteoporosis or osteopenia participates in a resistance exercise program, the risk of pathological fracture must be addressed. Osteoporosis, which is discussed in greater detail in Chapter 11, is a systemic skeletal disease characterized by reduced mineralized bone mass that is associated with an imbalance between bone resorption and bone formation, leading to fragility of bones. In addition to the loss of bone mass, there also is narrowing of the bone shaft and widening of the medullary canal.^9,29,82,174

The changes in bone associated with osteoporosis make the bone less able to withstand physical stress. Consequently, bones become highly susceptible to pathological fracture. A pathological fracture (fragility fracture) is a fracture of bone already weakened by disease that occurs as the result of minor stress to the skeletal system.^{29,82,189,209} Pathological fractures most commonly occur in the vertebrae, hips, wrists, and ribs.^82,174 Therefore, to design and implement a safe exercise program, a therapist needs to know if a patient has a history of osteoporosis and, as such, an increased risk of pathological fracture. If there is no known history of osteoporosis, the therapist must be able to recognize those factors that place a patient at risk for osteoporosis.^{29,54,82,174,189} As noted in Chapter 11, postmenopausal women, for example, are at high risk for primary (type I) osteoporosis. Secondary (type II) osteoporosis is associated with prolonged immobilization or disuse, restricted weight bearing, or extended use of certain medications, such as systemic corticosteroids or immunosuppressants.

Prevention of Pathological Fracture

As noted earlier in the chapter, evidence of the positive osteogenic effects of physical activity that includes resistance training has been determined. Consequently, in addition to aerobic exercises that involve weight-bearing, resistance exercises have become an essential element of rehabilitation and conditioning programs for individuals with or at risk for, osteoporosis.^8,9,226,238 Therefore, individuals who are at risk for pathological fracture often engage in resistance training.

Successful, safe resistance training must impose enough load (greater than what regularly occurs with activities of daily living) to achieve the goals of the exercise program (which include increasing bone density in addition to improving muscle performance and functional abilities) but not so heavy a load as to cause a pathological fracture. Guidelines and precautions during resistance training to reduce the risk of pathological fracture for individuals with or at risk for osteoporosis are summarized in Box 6.13.^209,226,238

There are only a few instances when resistance exercises are contraindicated. Resistance training is most often contraindicated during periods of acute inflammation and with some acute diseases and disorders. By carefully selecting the appropriate type (mode) of exercise (static vs. dynamic; weight-bearing vs. nonweight-bearing) and keeping the initial intensity of the exercise at a low to moderate level, adverse effects from resistance training can be avoided.

Pain

If a patient experiences severe joint or muscle pain during active-free (unresisted) movements, dynamic resistance exercises should not be initiated. During testing, if a patient experiences acute muscle pain during a resisted isometric contraction, resistance exercises (static or dynamic) should not be initiated. If a patient experiences pain that cannot be eliminated by reducing the resistance, the exercise should be stopped.

Inflammation

Dynamic and static resistance training is absolutely contraindicated in the presence of inflammatory neuromuscular disease. For example, in patients with acute anterior horn cell disease (Guillain-Barré) or inflammatory muscle disease (polymyositis, dermatomyositis) resistance exercises may actually cause irreversible deterioration of strength as the result of damage to muscle. Dynamic resistance exercises are contraindicated in the presence of acute inflammation of a joint. The use of dynamic resisted exercise can irritate the joint and cause more inflammation. Gentle setting (static) exercises against negligible resistance are appropriate.

Severe Cardiopulmonary Disease

Severe cardiac or respiratory diseases or disorders associated with acute symptoms contraindicate resistance training. For example, patients with severe coronary artery disease, carditis, or cardiac myopathy should not participate in vigorous physical activities, including a resistance training program, nor should patients with congestive heart failure or uncontrolled hypertension or dysrhythmias.^7,8

After myocardial infarction or coronary artery bypass graft surgery resistance training should be postponed for at least 5 weeks (that includes participation in 4 weeks of supervised cardiac rehabilitation endurance training) and clearance from the patient's physician has been received.⁸

Definition and Use

Manual resistance exercise is a form of active resistive exercise in which the resistance force is applied by the therapist to either a dynamic or a static muscular contraction.

• When joint motion is permissible, resistance usually is applied throughout the available ROM as the muscle contracts and shortens or lengthens under tension.

• Exercise is carried out in anatomical planes of motion, in diagonal patterns associated with PNF techniques,^165,279 or in combined patterns of movement that simulate functional activities.

• A specific muscle may also be strengthened by resisting the action of that muscle, as described in manual muscle-testing procedures.^137,158

• In rehabilitation programs, manual resistance exercise, which may be preceded by active-assisted and active exercise, is part of the continuum of active exercises available to a therapist to improve or restore muscular strength and endurance.

There are many advantages to the use of manual resistance exercises, but there also are disadvantages and limitations to this form of resistance exercises. These issues are summarized in Box 6.14.

Guidelines and Special Considerations

The general principles for the application of resistance exercises discussed in the preceding section of this chapter apply to manual resistance exercise. In addition, there are some special considerations that are unique to manual resistance exercises that should be followed. The following guidelines apply to manual resistance exercise carried out in anatomical planes of motion and in diagonal patterns association with PNF.

Body Mechanics of the Therapist

• Select a treatment table on which to position the patient that is a suitable height or adjust the height of the patient's bed, if possible, to enhance use of proper body mechanics.

• Assume a position close to the patient to avoid stresses on your low back and to maximize control of the patient's upper or lower extremity.

• Use a wide base of support to maintain a stable posture while manually applying resistance; shift your weight to move as the patient moves his or her limb.

Application of Manual Resistance and Stabilization

• Review the principles and guidelines for placement and direction of resistance and stabilization (see Figs. 6.12 and 6.13). Stabilize the proximal attachment of the contracting muscle with one hand, when necessary, while applying resistance distally to the moving segment. Use appropriate hand placements (manual contacts) to provide tactile and proprioceptive cues to help the patient better understand in which direction to move.²⁶⁴

• Grade and vary the amount of resistance to equal the abilities of the muscle through all portions of the available ROM.

• Gradually apply and release the resistance so movements are smooth, not unexpected or uncontrolled.

• Hold the patient's extremity close to your body so some of the force applied is from the weight of your body not just the strength of your upper extremities. This enables you to apply a greater amount of resistance, particularly as the patient's strength increases.

• When applying manual resistance to alternating isometric contractions of agonist and antagonist muscles to develop joint stability, maintain manual contacts at all times as the isometric contractions are repeated. As a transition is made from one muscle contraction to another, no abrupt relaxation phase or joint movements should occur between the opposing contractions.

Verbal Commands

• Coordinate the timing of the verbal commands with the application of resistance to maintain control when the patient initiates a movement.

• Use simple, direct verbal commands.

• Use different verbal commands to facilitate isometric, concentric, or eccentric contractions. To resist an isometric contraction, tell the patient to "Hold" or "Don't let me move you" or "Match my resistance." To resist a concentric contraction, tell the patient to "Push" or "Pull." To resist an eccentric contraction, tell the patient to "Slowly let go as I push or pull you."

Number of Repetitions and Sets/Rest Intervals

• As with all forms of resistance exercise, the number of repetitions is dependent on the response of the patient.

• For manual resistance exercise, the number of repetitions also is contingent on the strength and endurance of the therapist.

• Build in adequate rest intervals for the patient and the therapist; after 8 to 12 repetitions, both the patient and the therapist typically begin to experience some degree of muscular fatigue.

Techniques: General Background

The manual resistance exercise techniques described in this section are for the upper and lower extremities, performed concentrically in anatomical planes of motion. The direction of limb movement would be the opposite if manual resistance were applied to an eccentric contraction. The exercises described are performed in nonweight-bearing positions and involve movements to isolate individual muscles or muscle groups.

Consistent with Chapter 3, most of the exercises described and illustrated in this section are performed with the patient in a supine position. Variations in the therapist's position and hand placements may be necessary, depending on the size and strength of the therapist and the patient. Alternative patient positions, such as prone or sitting, are described when appropriate or necessary. Ultimately, a therapist must be versatile and able to apply manual resistance with the patient in all positions to meet the needs of many patients with significant differences in abilities, limitations, and pathologies.

NOTE: In all illustrations in this section, the direction in which resistance (R) is applied is indicated with a solid arrow.

Reciprocal motions, such as flexion/extension and abduction/adduction, are often alternately resisted in an exercise program in which strength and balanced neuromuscular control in both agonists and antagonists are desired. Resistance to reciprocal movement patterns also enhances a patient's ability to reverse the direction of movement smoothly and quickly, a neuromuscular skill that is necessary in many functional activities. Reversal of direction requires muscular control of both prime movers and stabilizers and combines concentric and eccentric contractions to decrease momentum and make a controlled transition from one direction to the opposite direction of movement.

Manual resistance in diagonal patterns associated with PNF are described and illustrated in the next section of this chapter. Additional resistance exercises to increase strength, power, endurance, and neuromuscular control in the extremities can be found in Chapters 17 through 23. In these chapters many examples and illustrations of resisted eccentric exercises, exercises in weight-bearing (closed-chain) positions, and exercises in functional movement patterns are featured. Resistance exercises for the cervical, thoracic, and lumbar spine are described and illustrated in Chapter 16.

Upper Extremity

Shoulder Flexion

Hand Placement and Procedure

• Apply resistance to the anterior aspect of the distal arm or to the distal portion of the forearm if the elbow is stable and pain-free (Fig. 6.14) VIDEO 6.1 .

• Stabilization of the scapula and trunk is provided by the treatment table.

Shoulder Extension

Hand Placement and Procedure

• Apply resistance to the posterior aspect of the distal arm or the distal portion of the forearm.

• Stabilization of the scapula is provided by the table.

Shoulder Hyperextension

The patient assumes the supine position, close to the edge of the table, side-lying, or prone so hyperextension can occur.

Hand Placement and Procedure

• Apply resistance in the same manner as for extension of the shoulder.

• Stabilize the anterior aspect of the shoulder if the patient is supine.

• If the patient is side-lying, adequate stabilization must be given to the trunk and scapula. This usually can be done if the therapist places the patient close to the edge of the table and stabilizes the patient with the lower trunk.

• If the patient is lying prone, manually stabilize the scapula.

Shoulder Abduction and Adduction

Hand Placement and Procedure

• Apply resistance to the distal portion of the arm with the patient's elbow flexed to 90°. To resist abduction (Fig. 6.15), apply resistance to the lateral aspect of the arm. To resist adduction, apply resistance to the medial aspect of the arm.

• Stabilization (although not pictured in Fig. 6.15) is applied to the superior aspect of the shoulder, if necessary, to prevent the patient from initiating abduction by shrugging the shoulder (elevation of the scapula).

PRECAUTION: Allow the glenohumeral joint to externally rotate when resisting abduction above 90° to prevent impingement.

Elevation of the Arm in the Plane of the Scapula ("Scaption")

Hand Placement and Procedure

• Same as previously described for shoulder flexion.

• Apply resistance as the patient elevates the arm in the plane of the scapula (30° to 40° anterior to the frontal plane of the body).^55,73,182

Shoulder Internal and External Rotation

Hand Placement and Procedure

• Flex the elbow to 90° and position the shoulder in the plane of the scapula.

• Apply resistance to the distal portion of the forearm during internal rotation and external rotation (Fig. 6.16A).

• Stabilize at the level of the clavicle during internal rotation; the back and scapula are stabilized by the table during external rotation.

Alternate Procedure

Alternate alignment of the humerus (Fig. 6.16 B). If the mobility and stability of the glenohumeral joint permit, the shoulder can be positioned in 90° of abduction during resisted rotation.

Shoulder Horizontal Abduction and Adduction

Hand Placement and Procedure

• Flex the shoulder and elbow to 90° and place the shoulder in neutral rotation.

• Apply resistance to the distal portion of the arm just above the elbow during horizontal adduction and abduction.

• Stabilize the anterior aspect of the shoulder during horizontal adduction. The table stabilizes the scapula and trunk during horizontal abduction.

• To resist horizontal abduction from 0° to 45°, the patient must be close to the edge of the table while supine or be placed side-lying or prone.

Elevation and Depression of the Scapula

Hand Placement and Procedure

• Have the patient assume a supine, side-lying, or sitting position VIDEO 6.2 .

• Apply resistance along the superior aspect of the shoulder girdle just above the clavicle during scapular elevation (Fig. 6.17).

Alternate Procedures: Scapular Depression

To resist unilateral scapular depression in the supine position, have the patient attempt to reach down toward the foot and push the hand into the therapist's hand. When the patient has adequate strength, the exercise can be performed to include weight bearing through the upper extremity by having the patient sit on the edge of a low table and lift the body weight with both hands.

Protraction and Retraction of the Scapula

Hand Placement and Procedure

• Apply resistance to the anterior portion of the shoulder at the head of the humerus to resist protraction and to the posterior aspect of the shoulder to resist retraction.

• Resistance may also be applied directly to the scapula if the patient sits or lies on the side, facing the therapist.

• Stabilize the trunk to prevent trunk rotation.

Elbow Flexion and Extension

Hand Placement and Procedure

• To strengthen the elbow flexors, apply resistance to the anterior aspect of the distal forearm (Fig. 6.18). VIDEO 6.3

• The forearm may be positioned in supination, pronation, and neutral to resist individual flexor muscles of the elbow.

• To strengthen the elbow extensors, place the patient prone (Fig. 6.19) or supine and apply resistance to the distal aspect of the forearm.

• Stabilize the upper portion of the humerus during both motions.

Forearm Pronation and Supination

Hand Placement and Procedure

• Apply resistance to the radius of the distal forearm with the patient's elbow flexed to 90° (Fig. 6.20) VIDEO 6.4 to prevent rotation of the humerus.

PRECAUTION: Do not apply resistance to the hand to avoid twisting forces at the wrist.

Wrist Flexion and Extension

Hand Placement and Procedure

• Apply resistance to the volar and dorsal aspects of the hand at the level of the metacarpals to resist flexion and extension, respectively (Fig. 6.21) VIDEO 6.5 .

• Stabilize the volar or dorsal aspect of the distal forearm.

Wrist Radial and Ulnar Deviation

Hand Placement and Procedure

• Apply resistance to the second and fifth metacarpals alternately to resist radial and ulnar deviation.

• Stabilize the distal forearm.

Motions of the Fingers and Thumb

Hand Placement and Procedure

• Apply resistance just distal to the joint that is moving VIDEO 6.6 . Resistance is applied to one joint motion at a time (Figs. 6.22 and 6.23).

• Stabilize the joints proximal and distal to the moving joint.

Lower Extremity

Hip Flexion with Knee Flexion

Hand Placement and Procedure

• Apply resistance to the anterior portion of the distal thigh (Fig. 6.24) VIDEO 6.7 . Simultaneous resistance to knee flexion may be applied at the distal and posterior aspect of the lower leg, just above the ankle.

• Stabilization of the pelvis and lumbar spine is provided by adequate strength of the abdominal muscles.

PRECAUTION: If, when the opposite hip is extended, the pelvis rotates anteriorly, and lordosis in the lumbar spine increases during resisted hip flexion, have the patient flex the opposite hip and knee and plant the foot on the table to stabilize the pelvis and protect the low back region.

Hip Extension

Hand Placement and Procedure

• Apply resistance to the posterior aspect of the distal thigh with one hand and to the inferior and distal aspect of the heel with the other hand (Fig. 6.25).

• Stabilization of the pelvis and lumbar spine is provided by the table.

Hip Hyperextension

Patient position: prone.

Hand Placement and Procedure

• With the patient in a prone position, apply resistance to the posterior aspect of the distal thigh (Fig. 6.26).

• Stabilize the posterior aspect of the pelvis to avoid motion of the lumbar spine.

Hip Abduction and Adduction

Hand Placement and Procedure

• Apply resistance to the lateral and the medial aspects of the distal thigh to resist abduction (Fig. 6.27) and adduction, respectively, or to the lateral and medial aspects of the distal leg just above the malleoli if the knee is stable and pain-free.

• Stabilization is applied to the pelvis to avoid hip-hiking from substitute action of the quadratus lumborum and to keep the thigh in neutral position to prevent external rotation of the femur and subsequent substitution by the iliopsoas.

Hip Internal and External Rotation

Patient position: supine with the hip and knee extended.

Hand Placement and Procedure

• Apply resistance to the lateral aspect of the distal thigh to resist external rotation and to the medial aspect of the thigh to resist internal rotation.

• Stabilize the pelvis.

Patient position: supine with the hip and knee flexed (Fig. 6.28).

Hand Placement and Procedure

• Apply resistance to the medial aspect of the lower leg just above the malleolus during external rotation and to the lateral aspect of the lower leg during internal rotation.

• Stabilize the anterior aspect of the pelvis as the thigh is supported to keep the hip in 90° of flexion.

Patient position: prone, with the hip extended and the knee flexed (Fig. 6.29).

Hand Placement and Procedure

• Apply resistance to the medial and lateral aspects of the lower leg.

• Stabilize the pelvis by applying pressure across the buttocks.