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The 3 types of skeletal muscle fibres are: Red / Slow (Type I fibres, 'slow twitch fibres') Red / Fast (Type IIa fibres, 'fast oxidative fibres') White / Fast (Type IIb fibres, 'fast glycolytic fibres')
Within skeletal muscle there are three types of fiber: Type I fibers are also known as slow twitch fibers. They are red in colour due to the presence of large volumes of myoglobin and so oxygen and high numbers of Mitochondria. Due to this fact they are very resistant to fatigue and are capable of producing repeated low-level contractions by producing large amounts of ATP through an aerobic metabolic cycle.
Type IIa fibers are also sometimes known as fast oxidative fibres and are a hybrid of type I and II fibers. These fibers contain a large number of mitochondria and Myoglobin, hence their red colour. They manufacture and split ATP at a fast rate by utilising both aerobic and anaerobic metabolism and so produce fast, strong muscle contractions, although they are more prone to fatigue than type I fibers. Resistance training can turn type IIb fibers into type IIa due to an increase in the ability to utilise the oxidative cycle.
Type IIb Often known as fast glycolytic fibers they are white in colour due to a low level of myoglobin and also contain few mitochondria. They produce ATP at a slow rate by anaerobic metabolism and break it down very quicky. This results in short, fast bursts of power and rapid fatigue. As mentioned above, this type of fiber can be turned into type IIa fibers by resistance training. This is a positive change due to the increased fatigue resistance of type IIa fibers. These fibers are found in large quantities in the muscles of the arms.