In general, muscles are composed of two basic kinds of fibers, namely fast twitch and slow twitch fibers. Typically, fast twitch fibers are attributed with the ability to contract swiftly and with great force. Sprinting and lifting heavy weights are among the activities where these muscle fibers could be engaged. Training that works the fast twitch muscles potentially might help enhance muscle strength and capacity, thereby improving athletic performance.
Muscles usually comprise a mix of both slow and fast twitch fibers. Some parts of the body might have a greater or lesser number of the fast or slow twitch type of fibers depending on how the muscles in that particular part are used. Fast twitch fibers are also called type II fibers. They are classified as type II-a and II-b fibers. Type II-a, or intermediate fast twitch fibers, have the capacity to generate energy without using oxygen (anaerobic), as well as by utilizing oxygen (aerobic).
Fast twitch muscle fibers usually are larger in size and paler in color than slow twitch fibers. They contain enzymes that can burn glycogen, a form of carbohydrate kept in the muscles. This makes glucose available to the body when it needs energy for muscle contraction.
While fast twitch fibers may have the ability to generate great amounts of force or power quickly, they often tire sooner than slow twitch fibers. The latter have a rich blood supply that delivers nutrients and oxygen to produce energy required for muscle contraction. Slow twitch muscles tend to make a good amount of energy at a slower rate. Fast twitch muscle fibers are considered less proficient in energy utilization.
An aerobic activity that is repetitive, of low intensity, but continues for a longer duration, typically engages slow twitch fibers. Thus the slow twitch group of fibers could be used when running long distances, for example. Fast twitch muscle fibers normally come into play during anaerobic activities which can be more strenuous or need more power, perform for a shorter amount of time, and take up less oxygen. Fast twitch fibers could be employed in actions such as jumping or pitching a fastball.
Mostly, the proportion of fast and slow twitch muscles fibers that occur in an individual has a genetic basis. An intensive training program or workout may bring about a positive effect on muscle fibers over time. For instance, fast twitch fibers could be developed and enlarged with strength training or sprint training.