What's in a muscle?
Our muscles are made up of two main types of fibers, called slow-twitch and fast-twitch. Fast-twitch fibers allow rapid, forceful muscle contraction — the sort of contraction required for sprinting. Slow-twitch fibers contract more slowly, but they also tire less easily. Endurance athletes tend to have more slow-twitch fibers, while power athletes (including sprinters, throwers, and jumpers) tend to have more fast-twitch fibers — a difference that may reflect both their genetics and their training habits.