""Bolt is an outlier. He’s enormous," Weyand said. "Typically when you get someone that big, they can’t start."
That’s because muscle speed in animals is generally tied to their size. For example, rodents, being much smaller than elephants, can move their muscles much faster. The same holds true for human beings. Sprinters are short and have more fast-twitch muscle fibers, allowing them to accelerate quickly, but compromising their ability to run longer distances. Four hundred-meter runners, almost always taller, have the reverse composition of muscle fibers.
Bolt, though, combines the mechanical advantages of taller men’s bodies with the fast-twitch fibers of smaller men."
http://www.wired.com/wiredscience/2008/08/bolt-is-freaky/