Skeletal Muscle

Skeletal muscle is surrounded by a layer of connective tissue, which is called the fascia, or epimysium, of the muscle; it consists mainly of collagen fibers. The fascia is built up in the same way as the outer layer of a joint capsule. It provides a surface against which the surrounding muscles can glide, and it gives muscles their form. Thus a muscle and its fascia are anatomically and physiologically bound together. When a muscle is fatigued, inflamed, or injured, the muscle is shortened and resists any stretching, and the same happens to the fascia. In fact, damage to the fascia creates additional problems. An inflamed fascia may adhere to other fascia, which makes muscle movement difficult or impossible. Scar tissue forms, and the lack of mobility can become permanent. This is one of the major sources of chronic soft tissue dysfunction and pain.

A muscle is further made up of small cell bundles, the fasciculi. Each fasciculus is surrounded by a thin layer of connective tissue, the perimysium. In the perimysium—which is made up of both col-lagenous and elastic fibers—the nerve and blood vessels branch off before finally reaching the actual muscle fibers. Each fasciculus consists of a number of muscle fibers, or muscle cells. Each muscle fiber is surrounded by a very thin layer of connective tissue, which is called endomysium (Fig. 5-1).

The structure and function of muscle fibers is described thoroughly in textbooks on physiology. The rest of this section is a brief review.

A muscle fiber is composed of small structures called muscle fibrils or myofibrils. The fibrils lie in parallel and give the muscle cell a striated appearance. Fibrils are made up of smaller regularly aligned components called myofilaments, which are chains of protein molecules. The striated appearance is attributable to the presence of two types of myofilament: actin and myosin. When the muscle contracts, the actin filaments move longitudinally between the myosin filaments. As a consequence, the myofibrils shorten and thicken.

The connective tissue surrounding the muscle, the epimysium, extends and is continuous with the muscle's tendon. The muscles of the body have very different shapes (Fig. 5-2). When a muscle contracts, it produces a force, F, that affects the origin and insertion of the muscle equally but in opposite directions. A muscle and its fascia become shortened when a muscle is fatigued, inflamed, or injured, which may create a static force on tissues of both origin and insertion. If the shortened muscle is forced to stretch, the muscle creates warning pain and conveys the stretching stress to the tendons of both origin and insertion. The consequence is tendinitis, which is a symptom of tendons, muscles, and related soft tissues, including nerves, blood vessels, and fascia. This condition can become more

Figure 5-1 Anatomical configuration of the skeletal muscle.

serious in athletes when medication is used to block or suppress the warning pain signals.

The structure of the muscle exactly serves its function. For example, strap-shaped muscle is found in places where it is necessary to execute large ranges of movement quickly. Pinnate-shaped muscle can be found where movements over a small range but of great strength are required. To assess the effect of a muscle, the clinician must also know where it is attached in relation to the joint. The alignment of force of a muscle is dependent on its physiologic cross-section. The ability of a muscle to create a force to do work depends on two factors: its physiologic cross-section and its position in relation to the joint. This knowledge is very important in the treatment of muscular symptoms related to movement.

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