Fast twitch muscle fibers are tissues capable of creating intense but short bursts of power. Muscle tissue is generally broken down into two categories: fast twitch and slow twitch. Fast twitch fibers are sometimes called Type II muscle, and slow twitch fibers are known as Type I.
Fast and slow twitch muscle fibers fall into the category of skeletal muscle, the type of muscle responsible for voluntary movement dictated by the somatic nervous system. Muscle tissue enables movement by contracting, and motor neurons determine how fast or slow this contraction occurs. Both muscle tissue types possess the same capacity to generate power, but fast twitch muscle fibers are activated by neurons at a rate 10 times faster than slow twitch fibers. People with large muscles simply have a greater amount of fast twitch tissue, sometimes at the expense of endurance.
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Fast twitch muscle fibers tire out more quickly because of their reliance on anaerobic metabolism. The word anaerobic simply means “without air.” Anaerobic metabolism transforms glucose and other sugars into cellular energy without the benefit of oxygen. The breakdown of each glucose molecule generates adenosine triphosphate (ATP) and two pyruvate molecules. The vital ATP carries energy within cells, but the lack of oxygen inhibits the creation of more ATP. The pyruvate molecules then become lactic acid.
Lactic acid was long considered to be the sole reason for muscle soreness, but research has shown that it acts as a reserve fuel when the body grows weak from exertion. Lactic acid supplies the body with needed sugars, but when it builds up too quickly, it can cause fatigue. Rest allows lactic acid to be converted by the liver into energy-enabling glucose.
There are also two distinct subcategories of fast twitch muscle fibers known as Type IIa and Type IIb. Type IIa fibers use both aerobic and anaerobic respiration to create energy and are sometimes called intermediate fibers. Type IIb fibers are the classic fast twitch type, creating the greatest bursts of strength and speed while relying exclusively on anaerobic respiration
More heavily muscled athletes such as sprinters and weightlifters obviously have a greater distribution of fast twitch muscle fibers than endurance athletes such as distance runners and cyclists. The distribution of both muscle types is inherited through genes. Heredity can be defied, though, to some degree. Those wishing to develop greater fast twitch or slow twitch muscle can do so through specific types of exercise.