Muscles are divided into two categories: fast twitch and slow twitch. Fast twitch fibers, or type II muscle, generate energy 10 times faster than slow twitch fibers, but tire quickly due to anaerobic metabolism. Lactic acid is a reserve fuel, but can cause fatigue. More muscular athletes have a greater distribution of fast-twitch muscle fibers, which is inherited through genes but can be developed through exercise.
Fast twitch muscle fibers are tissues capable of creating intense but short bursts of power. Muscle tissue is generally divided into two categories: fast twitch and slow twitch. The fast twitch fibers are sometimes called type II muscle, and the 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 allows movement by contraction, and motor neurons determine how fast or slow this contraction occurs. Both types of muscle tissue have the same ability to generate energy, but fast-twitch muscle fibers are fired by neurons at a rate 10 times faster than slow-twitch fibers. People with big muscles simply have a greater amount of fast-twitch tissue, sometimes at the expense of stamina.
Fast twitch muscle fibers tire more quickly due to 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. Vital ATP carries energy within cells, but a lack of oxygen inhibits the creation of more ATP. Pyruvate molecules are converted to lactic acid.
Lactic acid was long considered the only reason for muscle soreness, but research has shown that it acts as a reserve fuel when the body is weakened by exertion. Lactic acid supplies the body with necessary sugars, but when it builds up too quickly, it can cause fatigue. Rest allows lactic acid to be converted by the liver into glucose for energy.
There are also two distinct subcategories of fast twitch muscle fibers known as Type IIa and Type IIb. Type IIa fibers use 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 force and speed and relying solely on anaerobic respiration.
More muscular 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. However, heredity can be challenged, up to a point. Those who wish to develop more fast twitch or slow twitch muscle can do so through specific types of exercise.
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