Skeletal muscle fibers do not all have the same mechanical and metabolic characteristics. Different types of fibers can be identified on the basis of their maximal velocities of shortening, fast or slow, and the major pathway used to form ATP—oxidative or glycolytic. Fast and slow fibers contain forms of myosin that differ in the maximal rates at which they split ATP. This, in turn, determines the maximal rate of crossbridge cycling and thus the maximal shortening velocity. Fibers containing myosin with high ATPase activity are classified as fast fibers, and those containing myosin with lower ATPase activity are slow fibers. Although the rate of cross-bridge cycling is about four times faster in fast fibers than in slow fibers, the force produced by both types of crossbridges is about the same.
The second means of classifying skeletal muscle fibers is according to the type of enzymatic machinery available for synthesizing ATP. Some fibers contain numerous mitochondria and thus have a high capacity for oxidative phosphorylation. These fibers are classified as oxidative fibers. Most of the ATP produced by such fibers is dependent upon blood flow to deliver oxygen and fuel molecules to the muscle. Not surprisingly, therefore, these fibers are surrounded by many small blood vessels. They also contain large amounts of an oxygen-binding protein known as myoglobin, which increases the rate of oxygen diffusion within the fiber and provides a small store of oxygen. The large amounts of myoglobin present in oxidative fibers give the fibers a dark-red color, and thus oxidative fibers are often referred to as red muscle fibers. In contrast, glycolytic fibers have few mitochondria but possess a high concentration of glycolytic enzymes and a large store of glycogen. Corresponding to their limited use of oxygen, these fibers are surrounded by relatively few blood vessels and contain little myoglobin. The lack of myoglobin is responsible for the pale color of glycolytic fibers and their designation as white muscle fibers.
To sum up, three types of skeletal muscle fibers can be distinguished: 1) slow-oxidative fibers (type I) combine low myosin-ATPase activity with high oxidative capacity; 2) fast-oxidative fibers (type IIa) combine high myosin-ATPase activity with high oxidative capacity; 3) fast-glycolytic fibers (type IIb) combine high myosin-ATPase activity with high glycolytic capacity.