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The musculoskeletal system is designed primarily for locomotion, movement, and performance of functional physical tasks and to mechanically support and protect the body's organs. The system consists of muscles and tendons, ligaments, bones, joints, intervertebral discs, and their associated tissues such as synovial capsule, cartilage, fascia, and other fibrous tissues. However, these tissues should not be considered in isolation—their function depends on complex neurologic, biomechanical, and physiologic interactions. This chapter will discuss the biology of musculoskeletal tissues and their responses to injury and rehabilitation, and will introduce the complex interactions between the musculoskeletal system and the rest of the body as whole. Finally, an overview of common musculoskeletal ailments from a variety of body parts will be presented as examples of these principles.

A more detailed review will be provided in “Section III: Ambulatory Care: Sports, Musculoskeletal, and Pain Medicine” of this textbook.

Skeletal muscle comprises 40 to 45% of the total body mass, converting chemical energy derived from food intake into generation of mechanical force.1 Muscles also contribute to essential body functions such as generation of heat; blood glucose regulation; and storage of lipids, carbohydrates, and amino acids.2 Muscles typically attach to bones with strong tendons, the fibers of which branch deep within the muscle bellies. Tendons consist of bundles of collagen that form sequentially larger fascicles. These are often covered by synovial tissues that bathe the tendon in a thin layer of tenosynovial fluid for lubrication and nutrient delivery. Others are covered only by muscular tissue or a dense sheath of connective tissue known as peritenon. Most muscular strain injuries occur at the muscle–tendon junctions. These can be toward the end of the muscle belly where it tapers to become predominantly tendon tissue or deep within the muscle along the tendon fibers. Tendons themselves are also commonly injured at the bony attachments (entheses) and within areas of particular high stress (i.e., where they wrap around bony prominences) or in areas of low vascularity such as the mid-substance of the Achilles tendon.

Ligamentous tissues connect bone to bone. In the process, they form joint capsules, provide stability, and assist with energy conservation. Examples include knee collaterals, which are firm and minimally elastic, providing structure stability, and the shoulder capsular (glenohumeral) ligaments, which are thin and pliable and allow for a large range of motion. The iliofemoral (“Y”) ligament provides support in standing and therefore allows reduced muscular activity. It controls external rotation in flexion and both internal and external rotation in extension; this stores energy as the hip goes into extension, allowing for more efficient gait.3

Cartilaginous tissue cushions the spaces between bones and provides a smooth surface to reduce friction within joints. Two major cartilage types are hyaline and fibrocartilage. Both are matrices consisting of living cells (chondrocytes); a nonliving framework supports the chondrocytes and provides for ...

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