Type 2 fibres
Type 2 fibres are also known as fast twitch fibres. They are white in colour, contract rapidly but are quickly tired. These fibres are fast and strong and are suited to anaerobic type work.
Type 2 fibres can be divided into 2a and 2b. Type's 2a fibres are called intermediate fast-twitch fibres and they can use both aerobic and anaerobic pathways almost equally to create energy. In this way they are a combination of Type 1 and Type 2 muscle fibres.
Type 2b fibres use anaerobic metabolism to create energy and are the 'classic' fast twitch muscle fibres that excel at producing quick, powerful bursts of speed. This muscle fibre has the highest rate of contraction (rapid firing) of all the muscle fibre types, but they also have a much faster rate of fatigue and can't last as long before they need rest.
All individuals have a combination of all types of muscle fibres in their muscles. However, the exact composition of types will vary from individual to individual and is genetically determined. Different parts of the body will also have different combinations of muscle fibre types.
Our genetic make up may well influence what sports we are naturally good at. Top class athletes tend to fall into sports that match their genetic predispositions. For example world class sprinters possess about 80% fast twitch fibres whilst marathon runners tend to possess about 80% slow twitch fibres.
Fibres cannot change their type during life, however, they can be trained, so type 1 fibres can have some properties of type 2, or vice versa. 2a fibres can easily be trained to either favour aerobic exercise (long distance running), or anaerobic exercise (sprinting), or a combination of both (football player).
