This is an excerpt from Practical Guide to Exercise Physiology by Robert Murray & W. Larry Kenney.
Not surprisingly, there are different types of muscle cells, a characteristic that enables humans to perform explosive movements of short duration as well as complete amazing feats of endurance exercise. The muscle fiber (cell) types are simply referred to as type I (slow twitch) and type II (fast twitch). Type I fibers are better suited for endurance exercise and type II fibers are better suited for sprints or other brief, powerful movements. Figure 1.5 shows a cross-section of muscle stained to show the different fiber types. Motor units contain only one fiber type. The motor neurons that innervate type I motor units are smaller in diameter than the neurons that supply type II motor units. In addition to that difference, type I motor units contain fewer fibers than type II motor units. As a result, type II motor units develop more force when they are activated.
Muscle cells are called on to accomplish all sorts of tasks, so it should be no surprise that cells are specialized for distinct functions.
Micrograph reprinted from W.L. Kenney, J.H. Willmore, and D.L. Costill, 2015, Physiology of sport and exercise, 6th ed. (Champaign, IL: Human Kinetics), 39. By permission of D.L. Costill.
Most muscles are roughly 50% fast-twitch and 50% slow-twitch, but these proportions can vary widely, as shown in table 1.1. Some elite distance runners have leg muscles in which over 90% of the muscle cells are slow-twitch (type I) fibers, while some elite sprinters have the opposite mix. Although the ratio of fiber types is determined by genetics, proper training can improve the function of any muscle cell - the very basis for greater fitness and performance.
Arms and legs contain a similar proportion of type I and type II muscle cells, although those proportions vary from person to person.
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