Bolt might have never tried to run a mile and there's a crazy reason behind it. Physiology states everyone is born with two types of muscle fibers: type I or slow-twitch and type II or fast-twitch. Fast-twitch muscle fibers are large and powerful, but not very efficient. Slow-twitch muscle fibers are the opposite while smaller and relatively weaker than fast-twitch fibers, they're efficient, with great endurance. -think of the wiry frame of a typical marathon runner. Sprinters like Bolt typically have 65-85 percent fast-twitch muscle fibers. If Bolt were to train by running longer distances-even one mile-his body might begin to convert some fast-twitch fibers to slow-twitch, and his legendary sprinting speed would suffer.