Muscle Fiber Types and Their Physiological Role

Although all striated muscles contract, the time course of contraction in the living animal varies with the muscle fiber type (fast twitch, slow twitch), substrate (ATP, creatine phosphate, protein, fatty acid, or glycogen), and the amount of oxygen available. Slow-twitch fibers (Type I fibers or red fibers) are mainly postural and are dominant in aerobic or endurance activity, whereas fast-twitch fibers (TVpe IIA and IIB fibers) are mainly fast acting and are involved in anaerobic activity. Type IIB muscles can take on a greater aerobic role than Type IIA muscles. Most body muscles consist of different proportions of all types, although instances of pure Type IIA (m. cutaneous trunci) or Type I (m. masseter) do exist in various parts of the body (7).

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