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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.

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