Muscle fibers that must remain active over a long time (type I or slow-twitch fibers) are rich in mitochondria, whose iron-containing cytochrome oxidase enzymes and abundant myoglobin content give the fibers their red appearance. These type I fibers stain darkly for enzymes of the oxidative pathway, such as succinic acid dehydrogenase (SDH), but they do not have to generate high tensions and thus do not stain deeply for myofibrillar adenosine triphosphatase (ATPase) and glycolytic enzymes. Type I fibers, which tend to be small, are used in fine manipulations. They are the first fibers in any muscle to be activated when a low level of power is required. Because of their mechanical properties, they are called slow-twitch, fatigue-resistant (S) fibers. Energy is conserved by S fibers by a slow rate of relaxation following a twitch, and thus they require a low frequency of stimulation for the twitches to fuse into a sustained contraction (tetanus).

Compared with type I fibers, type II fibers, which must generate high tensions rapidly but need not remain active for prolonged periods, are relatively poor in mitochondrial enzymes and are white in appearance. On the other hand, they are rich in ATPase and glycolytic enzymes. Because of the mechanical properties, they are called fast-twitch, fatigable (FF) or type 2b fibers.

In recent years, it has become clear that some fibers have mechanical properties intermediate between those of S and FF fibers. They can generate a relatively fast twitch but are still fatigue resistant and are therefore called fatigue-resistant (FR) or type 2a fibers. Most muscles consist of a mixture of these three types of fibers, but the proportions vary depending on each muscle’s function and pattern of stimulation (i.e., training).

Muscles can respond to exercise patterns by an appropriate shift in their metabolic characteristics. Isometric, anaerobic exercise results in an increase in the number of myofibrils and in the amount of contractile protein per fiber. Either other fiber types are converted to FF fibers or a higher actomyosin ATPase and glycolytic enzyme activity occurs for all fiber types in the muscle. Thus, muscles of weight lifters and sprinters contain a high proportion of FF fibers. In contrast, aerobic exercise such as long-distance running and swimming induces the reverse enzymatic pattern and increases the muscle’s ability to use oxygen. The proportion of FR and S fibers is larger in marathon runners than in other athletes. It is still unclear whether the fiber types actually change as a consequence of training or whether marathon runners choose this sport because of their muscle fiber composition.