It might not seem like it to many of us, but the human body was built to run. Not in sprints over short distances, but over long distances, such as in marathons. In fact, humans can outrun any animal in a marathon. Well, nearly any animal that is.
Most animals can outrun humans in a sprint. Having four legs over two has its advantages. But over long distances humans can outrun nearly every other animal on land. What gives us this ability? Sweat does. When we sweat while running it allows the body to cool. Most other animals cool themselves by panting, which is a fairly inefficient way to cool. Even animals that can sweat, such as horses, would find themselves in second place to a fit human running in a marathon.
So why did evolution give humans this amazing ability to run long distances, effectively allowing humans to outrun any animal over long distances? Most scientists believe it has to do with hunting, and they have studied the Bushmen in Africa for supportive proof. Before slingshots and spears, hunting was extremely difficult. Many scientists hypothesize that the way that humans hunted down animals was by chasing them and wearing them down. By doing that it was not done as individuals, but as a group. Hunting an animal for several hours would cause it to retire from exhaustion, making the final kill relatively simple.
But even the structure of our feet, and even our butt have evolved to allow humans to outrun any animal. The short toes, spring like tendons and ligaments allow the body to conserve energy, effectively aiding in propelling the body forward. As for our butt, it is only ever really used once a person is running. So your butt was built for running, not sitting.