https://www.google.com/search?Fast twitch, slow twitch wrote:
Sprinters have a higher percentage of fast twitch muscle fibers. There are two types of fast twitch fiber, described as "white fiber" and "brown fiber". The white fibers are the fastest and will change very little with distance training. The brown fibers are not quite as fast, but with distance training they can "convert" over to behave a lot like slow twitch fibers. My guess is that great 800m runners have a higher percentage of brown fast twitch fibers.
Fast twitch muscle has a different metabolism than slow twitch. You can run an entire 100m race hardly even breathing, because those fibers don't require oxygen but they then fatigue very quickly.
"Brown and white fibers"
Interesting results
Very evidence, much science