e divided into two general categories basedon how quickly they contract. - Slow-twitch muscle fibers (also called slow oxidative or type I muscle fibers) contain relatively large amounts of mitochondria and are surrounded by more capillaries than fast-twitch fibers. ¥ As the name implies, slow-twitch fibers contract more slowly than fast-twitch fibers. They have lower force outputs, but are more efficient and fatigue- resistant than fast-twitch fibers. - Fast-twitch muscle fibers (also called type II muscle fibers) are further subdivided into fast-glycolytic (type IIx) and fast-oxidative glycolytic (type IIa) fibers. ¥ Type IIx muscle fibers contain a relatively small amount of mitochondria, have a limited capacity for aerobic metabolism, and fatigue more easily than slow-twitch fibers. They have considerable anaerobic capacity, and are the largest and fastest, and are capable of producing the most force, of all the skeletal muscle fibers. ¥ Type IIa muscle fibers possess speed, fatigue, and force-production capabilities somewhere between type I and type IIx fibers. For this reason, type IIa fibers are also called intermediate fibers.