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muscle fibre type

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The classification of muscle fibres according to their contractile and metabolic characteristics. The simplest classification is into slow-twitch (or type I) and fast-twitch (or type II) fibres although some classifications subdivide these two types further. Slow-twitch fibres generate energy mainly aerobically, shorten slowly, and are able to produce lesser force, but are fatigue-resistant. Fast-twitch fibres generate energy mainly anaerobically, shorten more quickly, and are able to produce greater force, but fatigue more rapidly. Muscles are composed of both types of fibre but the proportion varies between muscles and between individuals. The average adult has about an equal proportion of each type of fibre. Elite endurance runners typically have 70 per cent or more slow-twitch fibres in the leg muscles while sprinters have a predominance of fast-twitch fibres. Differences between individuals are mainly genetically driven.

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