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There are two types of muscle fibers, Type I, or slow twitch, and Type II, or fast twitch. Olympic-level athletes tend to participate in sports that match their genetic makeup. Olympic sprinters have been shown to possess about 80 percent fast twitch fibers, while marathoners tend to have 80 percent slow twitch fibers.
Type I muscle fiber is also known as slow twitch muscle fiber. Muscle fiber types can be broken down into two main types: slow twitch (Type I) muscle fibers and fast twitch (Type II) muscle fibers.
Type II muscle fiber is also known as fast twitch muscle fiber. Muscle fiber types can be broken down into two main types: slow twitch (Type I) muscle fibers and fast twitch (Type II) muscle fibers. These fast twitch fibers can be further categorized into Type IIa and Type IIb fibers, which are also known as "fast twitch oxidative" and "fast twitch glycolytic," respectively. Type I fibers are characterized by low force/power/speed production and high endurance, Type IIB fibers are characterized by high force/power/speed production and low endurance, while Type IIA fall in between the two.
It is possible that a fiber might be transformed from Type IIB to Type IIAB to Type IIA with exercise training. Furthermore, researchers at Boston University School of Medicine have found that increasing the mass or size of type II muscle fibers will lead to a significant decrease in fat mass or the amount of fat in the body. A new study in the February issue of Cell Metabolism suggests that in regards to weight loss, lifting weights may be just as important as running on the treadmill.