If you think all it takes to be a great long distance runner is a good cardiovascular system and strong leg muscles, think again. Equally important are well-developed butt muscles. Not only will you look great in those running shorts but you will be able to run faster for longer and avoid painful injuries. Actually, butt muscles (and more specifically gluteus maximus) developed into a bigger size to become strong enough to allow humans to run upright, opposite to monkeys.