In running, you've all seen the Sprinter and the Marathoner. One looks like an 80s movie character, and the other, like he's had too many crash-course diets. They're both runners, so how come they don't look alike? Simple. They practice different sports. It's explosive strength versus endurance. The answer is in the way these athletes train their muscles. What you need to know about muscles is that they contain different fibers, fast twitch and slow twitch. A Sprinter will primarily train his fast twitch fibers. These react instantly and work super fast, but quickly run out of power. When trained, fast twitch fibers naturally become big and heavy. On the other hand, a Marathoner uses mostly his slow twitch fibers, not as reactive and speedy as their fast twitch brothers, but they can work nonstop for hours. Even when trained, these fibers remain slim and light. Okay, the running capability of both athletes don't only depend on the physique of their legs. The upper body is also important. For the Sprinter, to be able to burst from the starting line and to maintain a good rhythm most running, he needs balance, so his arms should have a considerable size as well. So he works out his upper body in the gym. That's why the Sprinter looks very muscular. And because the Sprinter only has to perform for a couple of seconds, the added weight is trivial. The Marathoner is a different story. Every pound in his body has to be carried around for a total of 42 kilometers. He gains a lot of benefits by being light, so all excessive muscle mass and fat has to be shed in order to be a top marathoner. That's why he looks so skinny. On top of the difference in training, genetics also play a part. A normal person has about an even amount of fast and slow twitch fibers, but the persons that make it to the top tend to have more of one kind. So even untrained, they wouldn't have the same physical appearance. The Sprinter and the Marathoner. Both runners. Yet different athletes.