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Lecture Notes

There are two broad categories of muscle fiber types in our bodies. The first is slow twitch and the second is fast twitch. The percentage of each in the muscles is mostly determined by genetics, and the average person has about 50/50, so about half slow twitch and half fast twitch. The slow twitch are fibers that are good for low-intensity aerobic activity, and even moderate-intensity aerobic activity, activity that requires the body to continually contract the muscles, and these slow twitch fibers are slower to contract, but they have a very high resistance to fatigue, meaning they can contract over and over again without fatigue, compared with the fast twitch fibers that are able to contract much more quickly, but also fatigue much more quickly. And so these fast twitch fibers are using glycogen and not oxygen for energy to fuel that contraction, whereas the slow twitch fibers are storing oxygen and using that oxygen to produce the movement. Importantly, slow twitch and fast twitch fibers are not mutually exclusive, so just like we talked about in cardiorespiratory activities or aerobic activities, that sometimes we use anaerobic systems of energy as well, especially at the beginning of aerobic activities. Fast twitch and slow twitch fibers are not mutually exclusive; both types work together to produce muscular movement.