ATP ADP lactic acid mitochondria Marathon runners have a much higher percentage of "slow twitch" fibers in their muscles, which make it possible for them to run long distances at a steady pace. On the other hand, sprinters' muscles are composed mostly of "fast twitch" fibers, which allow to allow for intense bursts of exertion. Therefore, marathon runners have more in their muscles to allow for greater production in long endurance Tuns.​