Skeletal muscle consists of muscle fibers, each of which is a multi-nucleated cylindrical cell. There are two kinds of skeletal muscle fibers: fast-twitch fibers (type II) and slow-twitch fibers (type I). Type II fibers are further classified into 3 categories: IIA, IID and IIB. Their speed of contraction is IIB>IID>IIA>I. Their common energy source is ATP. In fast-twitch fibers, ATP is generated by glycolysis, hence rich in glycogen granules. In slow twitch fibers, ATP is generated from oxidative phosphorylation, hence rich in mitochondria. Type IIA fibers depend on both. The fast-twitch fibers are used for quick movement, and slow-twitch fibers are used for sustained contraction. The number of fibers activated increases with exercise intensity. Type I fibers are the first to be recruited followed by type II fibers.

Sarcomere is a subcellular organelle, and defines the unit of contraction. It is composed of interdigitating thick and thin filaments. The major component of the thick filament is myosin with mw of ~500 kD. Additional components are C-protein and titin (also called connectin), which wrap around the thick filament. The major component of the thin filament is actin. The interaction between actin and myosin generates force, which is transmitted to the next sarcomere and eventually to a skeleton. The interaction is regulated by Ca²⁺ via tropomyosin (Tm) and troponin (Tn). There is a positive feedback mechanism called “cooperativity” between myosin, actin, Tm and Tn: Tm allosterically affects actin to improve actin-myosin interaction; the interaction of actin and myosin enhances Tm binding to actin, etc. Such a system turns on quickly, whereas it turns off in a caotic manner.

The molecular domain which projects from myosin to actin is called cross-bridges. With a repetitive attachment and detachment of cross-bridges, filament sliding occurs to generate force and perform work. This is called the cross-bridge cycle, and it goes through several intermediate steps. Each step is called an elementary step, which includes ATP-binding step, ATP-cleavage step, phosphate(Pi)-release step, and ADP-release step. The binding/release steps are preceded (or post-ceded) by a conformational change in the actomyosin complex. We now know that the conformational change which precedes Pi-release is the step that generates force.