A type of skeletal muscle fibre that produces relatively rapid, brief, and strong contractions but fatigues quickly. Fast-twitch fibres are characteristic, e.g., of muscles of the hand and eye, where rapid movements are required but are also found together with slow-twitch fibres in other muscles of the body. They contain a form of the contractile protein myosin that hydrolyses ATP very rapidly, causing fast interactions of the thick and thin filaments of the muscle fibre and rapid development of muscle tension. But the ATP cannot be replenished fast enough to sustain the rapid muscle contractions; hence fast-twitch fibres are suited for short-term work, as in the biceps of the upper arm. Oxidative fast-twitch fibres (type IIA) use aerobic metabolism to manufacture ATP; they contain the oxygen storage protein myoglobin, are well supplied with blood vessels and mitochondria, and occur as ‘red’ muscle. In contrast, glycolytic fast-twitch fibres (type IIB) rely on glycolysis to make ATP, have relatively few mitochondria and blood vessels, and make up ‘white’ muscle. The proportions of fast-twitch and slow-twitch fibres determine the performance characteristics of muscle; athletes with a predominance of fast-twitch fibres are more suited to sprint events, whereas those with more slow-twitch fibres tend to excel at endurance events, such as long-distance running. These proportions in any individual are largely the result of heredity, although training can have some influence.