Within skeletal muscles, there are three types of fibre. Type one (I), type two A (IIa) and type two B (IIb). Each fibre types has different qualities in the way they perform and how quickly they fatigue.
Type I muscle fibres
Type I fibre are also known as slow-twitch fibre. 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.
For this reason, the muscles containing mainly type I fibres are often postural muscles such as those in the neck and spine due to their endurance capabilities Also, athletes such as marathon runners have a high number of this type of fibre, partly through genetics, partly through training.
Type IIa muscle fibres
Type IIa fibres are also sometimes known as fast oxidative fibres and are a hybrid of type I and II fibres. These fibres 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 fibre. Resistance training can turn type IIb fibres into type IIa due to an increase in the ability to utilise the oxidative cycle.
Type IIb muscle fibres
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 quickly. This results in short, fast bursts of power and rapid fatigue. As mentioned above, this type of fibre can be turned into type IIa fibres by resistance training. This is a positive change due to the increased fatigue resistance of type IIa fibres. These fibres are found in large quantities in the muscles of the arms.