When scientists began to explore the different types of muscles, muscle fibers were categorized depending on their color -> Red and white muscle fibers. Because Red muscle fibers have more blood vessels and mitochondria, they appeared red and therefore were just called "red muscle fiber". The other type had a lower content of blood vessels and mitochondria appear white. White muscle fiber has the ability to grow up to four times bigger and contracts 2 to 3 times faster then red muscle fiber. That's why white muscle fiber is also called fast twitch muscle fiber and red fiber is also known as slow twitch muscle fiber. White (fast twitch) is what bodybuilders and short distance sprinters want to build. Red (slow twitch) which on the other hand can run on less energy, and that's why marathon runners and cyclists want white fiber.