This document discusses the structure and function of skeletal muscle. It describes the classification of muscles, the structure of muscle fibers including myofibrils and sarcomeres, and the molecular components actin and myosin that enable contraction. It explains that calcium triggers the interaction of actin and myosin heads, leading to muscle shortening. Energy for contraction comes initially from ATP and phosphocreatine, then from glycogen and oxidative metabolism. The document also distinguishes between isotonic contraction, where muscle length changes, and isometric contraction, where tension but not length changes.