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.
Type IIA Fibers
Type IIA fibers, or fast oxidative fibers, are used more during sustained power activities such as sprinting 400 meters or doing repeated lifts with a weight below maximum (but not with very light weights). They contain very large amounts of myoglobin, very many mitochondria and very many blood capillaries. Type II A fibers are red, unlike Type II B fibers, which are white. Type IIA fibers have a very high capacity for generating ATP by oxidative metabolic processes, and split ATP at a very rapid rate. They have a fast contraction 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. Type IIB fibres contain a low content of myoglobin, relatively few mitochondria, relatively few blood capillaries and large amounts glycogen. Type II B fibres are white, while Type I (slow twitch) fibers are red. Type IIB fibers are geared to generate ATP by anaerobic metabolic processes, however, they are not able to supply skeletal muscle fibres continuously with sufficient ATP, and fatigue easily.
ATP at a fast rate and have a fast contraction velocity. Such fibres are found in large numbers in the muscles of the arms. 
- ↑ 1.0 1.1 Elizabeth Quinn (October 30, 2007). Fast and Slow Twitch Muscle Fibers. About.com. Retrieved on 2008-05-13.
- ↑ Baggett, Kelly (n.d.). Understanding Muscle Fiber Types. Bodybuilding.com. Retrieved on 2008-10-17.
- ↑ University of Oregon (n.d.). Muscle Physiology. UOregon.edu. Retrieved on 2008-05-13.
- ↑ Nick H. (n.d.). Type I Vs Type II Muscle Fibers and Weight Lifting Programs. EzineArticles.com. Retrieved on 2008-05-13.
- ↑ Dr. Sanjukta Acharya (review) (5 February 2008). Type 2 muscle important in body metabolism and obesity. Rxpgnews.com. Retrieved on 2008-05-13.
- ↑ 6.0 6.1 Leyland, Tony (July 2008). "Human Power Output and Crossfit Metcon Workouts". CrossFit Journal (71).
- ↑ 7.0 7.1 Brian Mac (n.d.). Muscle Types. BrianMac.co.uk. Retrieved on 2008-05-13.
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