Fast twitch, Slow twitch

Every muscle in your body is made up of fibres. These fibres are one of 2 types: Fast twitch muscle fibres and slow twitch muscle fibres. Fast twitch muscle fibres can do a lot of work quickly, but also tire quickly, whereas slow twitch muscle fibres can't work as quickly, but have a lot more endurance. The average person has around 60% fast twitch and 40% slow twitch muscle fibres. You cannot change what kind of muscle fibres you have, so it is a huge advantage to sprinters, powerlifters, javelin throwers, shotputters etc. to have fast twitch muscles, while those who play endurance field sports like rugby, football, gaelic sports, basketball etc. or distance running would want different balances depending on the sport and their position in the sport. Since what muscle fibres you have is hereditary, it has often been theorized that some places have better chances of turning out certain types of athletes e.g. Jamaican sprinters.

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The idea that a particular body type can make people better at sprinting and other similiar sports helps explain how small, poor countries with probably not-very-good facilities can not only compete with huge developed nations like the USA, but beat them almost every time. A lot of work in Jamaica has been put into sprinting, and they probably got the idea to start competing in the sport due to the huge amount of natural talent available. Poverty in Jamaica means that kids see sprinting as their chance to make a break and get rich, and thus train very hard. Poverty may also have helped natural selection in the past, in tribal times when fast runners could get more food. It has been shown that descendants of slaves in America have more fast twitch muscle fibres than white people, probably a result of strong slaves being more valuable to their owners and therefore less likely to be killed. It is possible that the slave trade had similiar affects on Jamacian culture.