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Muscle Composition

Sprinters and endurance athletes differ in the composition and the capabilities of their muscles. These differences may be influenced by training decisions and genetic factors.

How it works

Our muscles are made of two main types of fibres: slow-twitch and fast-twitch. We move our bodies by contracting and releasing our muscles, and fibre types play a role how a muscle contracts. While slow-twitch fibres contract slowly, they tire less easily. Fast twitch fibres contract quickly, offering short bursts of powerful energy but tire quickly in comparison to slow-twitch muscles.

Training for a marathon? Your slow-twitch fibres help you go the distance. Always bringing your ‘A game' to sports like sprinting, throwing, and jumping? Your fast-twitch fibres help you bring maximum power right out of the gate.

The genetic link

While training and physical fitness influence athletic performance, scientific evidence suggests that genetics also influence prowess. Studies have found that most elite power athletes have a specific genetic variant in a gene related to muscle composition called the ACTN3 gene. This variant causes muscle cells to produce alpha-actinin-3, a protein found in fast-twitch muscle fibres.

Slow vs. fast twitch muscles

Did you know?

Almost all elite power athletes who have participated in genetic studies produce the alpha-actinin-3 protein. However, you don't need this protein to be physically fit. This genetic variant only explains about 2-3% of the difference in muscle performance between different people, and for most athletes that margin is too small to even notice. So, if you don't have this genetic variant, don't sweat it.

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Itching and twitching to learn about your muscle fibres? 23andMe's Health + Ancestry Service can tell you more. Pick up a kit to see what your genes have to say about your muscle composition.

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