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Type II Muscle Fiber

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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.[1] These fast twitch fibers can be further categorized into Type IIa and Type IIb fibers,[1] 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.[2]

It is possible that a fiber might be transformed from Type IIB to Type IIAB to Type IIA with exercise training.[3] 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.[4] 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.[5]


Type IIA Fibers Edit

Type IIA fibers, or fast oxidative fibers, are used more during sustained power activities such as velocity and are more resistant to fatigue than Type IIB. [6]

Type IIB FibersEdit

Type IIb fast-twitch fibers, or fast glycolytic fibres (also known as Type IIx), are recruited for very short-duration high-intensity bursts of power such as maximal and near-maximal lifts and short sprints.[7]


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