Medical Dictionary

Skeletal muscle fibers having high myofibrillar ATPase activity, high glycolytic enzyme activities, and an intermediate glycogen content which produce a fast twitch. There are two types. Fast fatigable fibers, also called white fibers, have a low myoglobin content, and a small mitochondrial content, and fatigue rapidly due to their limited glycogen content and low capacity for oxidative metabolism. Fast fatigue-resistant fibers, also called red fibers, have a large mitochondrial content and a high myoglobin content, related to their resistance to fatigue. (From Best, Physiological Basis of Medical Practice, 12th ed, p100)