Why Usain Bolt is fast and Lance Armstrong never stops

Some people have been given the gift of running insanely fast (Usain Bolt), while others have been given the ability to run for a really long time (your marathoners). The difference between the two is how many slow-twitch and fast-twitch muscle fibers they have. Fast twitch muscle fibers can contract reallly fast, allowing the person to fly down the track, or bench 300lbs. When you excersize your fast-twitch muscle fibers, they grow and grow, thats why some people bulk up really easily, they have lots of fast-twitch muscle fibers. But fast-twitch muscle fibers get tired extremely quickly. Thats where slow-twitch muscle fibers come in. They can keep of contracting for hours if you try hard enough, which is why you can walk around Busch Gardens all day and still not be too tired. But no matter how much you work them, they don't get much bigger, you normally end up just losing the fat around them, making them more defined. Most of us have a happy medium of muscle fibers, about half fast twitch and half slow twitch, which is perfect by evolutionary standards. Yay for muscle fibers!