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Is this composition determined entirely by genetics? Or can training a certain way altered the composition?

Can someone go from 70/30 fast/slow to 30/70 fast/slow?

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So basically you have three types of muscle fibre:

  • Type I (slow-twitch)
  • Type IIa (fast oxidative)
  • Type IIx (fast glycolytic)

Most people are generally born with roughly a 50/50 split between types I and II. Top level endurance athletes can have up to 90-95% type I fibres, whereas strength/power athletes can have 60-80% type II. It's generally believed that you can alter your makeup by around 10% either way (see this article).

It's well established that it's possible to convert e.g. IIa to IIx with training, but it's less certain that you can actually convert type I to II (or vice-versa). What you are able to do is to target certain fibres with exercise so that the proportion of Type I to II fibres is altered.

A fun bit of anecdotal evidence - the marathon runner Alberto Salazar aimed to do sufficient endurance exercise that he lost his ability to jump, as his fast-twitch fibres had reduced to such an extent.

See this review of the literature for more details

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