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 fibre 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] also this muscle fibers have myosin ATPase which are of high activity

Type IIA Fibers 

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.

Type IIB Fibers

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.[6]

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