The musculoskeletal structures involved with human motion include the nerves, muscles, and tendons; the fascia and ligaments that provide support and stability; and the joints around which the motions occur. The neural structures are discussed in Chapter 2. The musculoskeletal system functions intimately with nervous tissue to produce coordinated movement and provide adequate joint stabilization and feedback during sustained positions and movements.
The basic components of tissue include the following:
Collagen. Provides tensile strength of tissue. Over 20 types have been identified, of which type I and type II are the most abundant. Bone, ligaments, tendons, meniscus, and skin are all primarily type I collagen, while articular cartilage is primarily composed of type II collagen.
Elastin. Provides elasticity and deformation to tissue.
Proteoglycans. Provide the binding properties of tissue.
Extracellular matrix. Formed by a combination of collagen, elastin, proteoglycans, and inorganic components.
Cells. Three major types of cells exist in musculoskeletal tissue: blasts (responsible for tissue formation, e.g., osteoblast, fibroblast), cytes (responsible for the maintenance of tissue and the response of tissue to stress, e.g., osteophytes, fibrocytes), and clasts (responsible for resorption of tissue, e.g., osteoclast, fibroclast).
There are approximately 430 muscles in the body, each of which can be considered anatomically as a separate organ. Of these 430 muscles, about 75 pairs provide the majority of body movements and postures.1 Muscle (Figure 1-1) may be classified functionally as either voluntary or involuntary and structurally as either smooth, striated (skeletal), or cardiac (Table 1-1).
Smooth and cardiac muscles can contract without nervous stimulation, but their contraction is influenced by the nervous system. Skeletal muscles cannot contract unless stimulated by neurons. Therefore, in the presence of weakness, the clinician must include nerve (spinal or peripheral) injury in the differential diagnosis.