I've read that it's a lot more to do with culture than genetics, I'm not sure of how true that is. Although many dark skinned people, in particular Jamaicans, have a high percentage of fast twitch muscles, it's been proven that 60% or so of white people also have this type of muscles. However, apparently the difference is that running is BIG in Jamaica. They have top training facilities and the belief that they can win, because they have so many top athletes competing on the world scene. There aren't many other sports that they compete in, generally, so all the top athletic types, rather than going into other sports, go into running. Parents and families see running as a way to get to the top, so they're hugely supportive, and kids see it as a way to become a superstar and have a better life, so they train like hell. And they have a very supportive coaching structure so that athletes get all the help they need to make it.
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