The skeletal muscles, like the joints, are designed to contribute to the body's needs for mobility and stability. Muscles serve a mobility function by producing or controlling the movement of a bony lever around a joint axis; they serve a stability function by resisting extraneous movement of joint surfaces and through approximation of joint surfaces. The body is incapable of either supporting itself against gravity or producing motion without muscle function.
Human movement is a complex interaction of muscle function and joint lever systems under the control of the nervous system. Daily, clinicians evaluate the muscle function of patients in order to determine the extent of the loss of muscle function and to formulate appropriate interventions to help the patient regain muscle function. Understanding muscle function begins with a clear picture of the structure of a muscle from the contractile proteins within each muscle fiber to the organization of the fibers in the whole muscle. From the muscle structure, we examine the basic mechanical properties of muscle fibers, whole muscles, and groups of muscles. To complete our understanding of muscle function, we analyze the function of muscles working across joints to produce the intricate movements we use for daily activities, work, sport, and play. Unfortunately, some of the movements used in these activities may cause injury to the muscles and tendons. The following case identifies a common muscle injury. Throughout this chapter, you will see how the structure and function of the muscles can be applied to this clinical situation.
Case 3-1: Patient Case
Vik Patel, a 50-year-old man, was playing softball one summer evening. He was trying to catch a fly ball when he stepped back with his right foot and slipped slightly. As his foot slipped, the motion at the ankle was dorsiflexion, and the ankle plantar flexor muscles were contracting as he tried to push off so that he could run forward. At the moment he tried to push off, he felt a twinge of pain in his right calf muscle. Vik states that he has pain in the calf muscle and along the Achilles tendon when he tries to stand on his toes and when he does calf-stretching exercises. After an evaluation of Vik, it appears that he may have strained the calf muscle or caused some tendinitis.
ELEMENTS OF MUSCLE STRUCTURE
Skeletal muscles are composed of muscle tissue (contractile) and connective tissue (noncontractile). The muscle tissue has the ability to develop tension in response to chemical, electrical, or mechanical stimuli. The connective tissue, on the other hand, develops tension in response to passive loading.1 The properties of these tissues and the ways in which they are interrelated give muscles their unique characteristics.
Composition of a Muscle Fiber
A skeletal muscle is composed of many thousands of muscle fibers. A single ...