All fibers in a particular motor unit are either slow - twitch or fast - twitch fibers, each of which has distinctive metabolic and contractile capabilities. Slow - twitch fibers are also referred to as type I fibers. They are more resistant to fatigue than are fast - twitch fibers; however, the time required to generate force is much greater in slow - twitch fibers. Because they are relatively fatigue resistant, slow - twitch fibers are associated primarily with long - duration, aerobic - type activities.
Fast - twitch fibers (also referred to as type II fibers) are capable of producing quick, forceful contractions but have a tendency to fatigue more rapidly than do slow - twitch fibers. Fast - twitch fibers are useful in short -term, high - intensity activities, which mainly involve the anaerobic system. Fast — twitch fibers are capable of producing powerful contractions, whereas slow - twitch fibers produce a long - endurance type of force. There are two subdivisions of fast - twitch fibers. Although both types of fast - twitch fibers are capable of rapid contraction, type IIa fibers are moderately resistant to fatigue whereas type IIb fibers fatigue rapidly and are considered the "true" fast - twitch fibers.
Within a particular muscle are both types of fibers, and the ratio in an individual muscle varies with each person. Those muscles whose primary function is to maintain posture against gravity require more endurance and have a higher percentage of slow — twitch fibers. Muscles that produce powerful, rapid, explosive strength movements tend to have a much greater percentage of fast - twitch fibers. Because this ratio is genetically determined, it may play a Large role in determining ability for a given sport activity. Sprinters and weight lifters, for example, have a large percentage of fast - twitch fibers in relation to slow - twitch fibers. Conversely, marathon runners generally have a higher percentage of slow - twitch fibers.
The metabolic capabilities of both fast - twitch and slow - twitch fibers may be improved through specific strength and endurance training. It now appears that there can be an almost complete change from slow - twitch to fast - twitch and from fast - twitch to slow - twitch fiber types in response to training.
