Muscles are composed of thousands of individual muscle fibres, which are held together by connective tissue. However, muscle fibres differ in physiological makeup. Skeletal muscle has two main fibre types: slow twitch and fast twitch. Fast twitch and slow twitch fibres vary in different muscles and in different individuals – these proportions tend to be genetically determined.
As the fibres have distinct characteristics, this will affect performance in certain sporting activities. For example, a marathon runner may have almost 80% slow twitch fibres which are designed for long periods of low intensity work; whereas, a sprinter may have 80% fast twitch fibres, which can generate extremely large forces, but fatigue easily.
Each muscle contains both types of fibres, but not in equal proportions. The fibres are grouped in motor units, with only one type of fibre in any given unit.