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Skeletal Muscle Contains Slow (Red) & Fast (White) Twitch Fibers

المؤلف:  Peter J. Kennelly, Kathleen M. Botham, Owen P. McGuinness, Victor W. Rodwell, P. Anthony Weil

المصدر:  Harpers Illustrated Biochemistry

الجزء والصفحة:  32nd edition.p628

2026-01-19

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Different types of fibers have been detected in skeletal muscle. One classification subdivides them into type I (slow twitch), type IIA (fast twitch-oxidative), and type IIB (fast twitch glycolytic). For the sake of simplicity, we shall consider only two types: type I (slow twitch, oxidative) and type II (fast twitch, glycolytic). The type I fibers contain myoglobin and mitochondria; their metabolism is aerobic, and they maintain relatively sustained contractions. The white type II fibers lack myoglobin and contain few mitochondria: they derive their energy from anaerobic glycolysis and perform contractions of relatively short duration.

The proportion of these two types of fibers varies among the muscles of the body, depending on function and training. For example, the number of type I fibers in certain leg muscles increases in athletes training for marathons, whereas the number of type II fibers increases in sprinters. Type I fibers rely heavily on aerobic metabolism to regenerate ATP, while type II fibers rely on oxygen-independent sources such as creatine phosphate. For this reason, many endurance athletes will engage in carbohydrate loading, the ingestion of meals that predominantly consist of foods with a high starch content, in an effort to increase the size of their glycogen stores, while some sprinters will attempt to increase the pool of creatine phosphate in their muscles by ingesting creatine as a dietary supplement.