Born That Way
Some people are born with more fast-twitch muscle fiber than others, and that's a big part of what makes some people so much faster than others.
Muscles are made of two kinds of fibers--fast twitch and slow twitch.
Slow-twitch fibers use energy aerobically and can work for long periods of time before getting tired. They contract more slowly, but can keep going without burning themselves out.
Fast-Twitch vs. Slow-Twitch
Slow-twitch fibers have a lot of myoglobin, a protein that helps transfer oxygen into the mitochondria, the power houses of the cell that produce cellular energy.
So marathon runners must have a lot of slow-twitch muscle fibers, and sprinters must have a lot of fast-twitch muscle fiber.
Fast-twitch fibers consume energy quickly and are good for short, powerful bursts of movement such as sprinting and jumping. World-class sprinters typically have about 80% fast-twitch muscle fiber.
And of course they train incredibly hard to develop their natural gifts. Olympic gold is not won by muscle fiber alone. But it certainly helps.
"Fast and Slow Twitch Muscle Fibers" (About.com)