Sports Medicine

Medicine > Anatomy & Physiology > Muscle Fibres

Fast and slow twitch muscle fibres

Have you ever wondered why some people are better long-distance runners and others better sprinters? It is because our muscles are made up of two main types of fibres - slow-twitch and fast-twitch. Not all muscles in the same person are one type or the other, and particular muscles such as postural muscles are always slow twitch some they can serve their function.

Slow-Twitch

Slow-twitch fibres contract slowly and can be used for longer periods of time. Postural muscle of the back are composed of mostly slow-twitch muscle fibres. Athletes who run long distances need more slow-twitch fibres in their legs. Slow-twitch muscle fibres rely on oxygen as their main energy source.

Fast-Twitch

Fast-twitch fibres contract quickly and provide strength and speed, though they also fatigue more quickly. Sprinters require short but intense bursts of energy and therefore need more fast-twitch muscle fibres. Fast-twitch muscle fibres rely on ATP and glycogen as their main energy source. As ATP sources can be rapidly depleted, lactic acid is a by product of the breakdown of glycogen, fast-twitch fibres can only be active for a short period of time.

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