At some point in the future, if mankind hopes to settle planets outside our solar system, it will be crucial to determine the range of planetary conditions under which human beings could survive and function. In this article, we apply physical considerations to determine the limitations that gravity imposes on several systems governing the human body. Initially, we examine the ultimate limits at which the human skeleton breaks and muscles become unable to lift the body from the ground. We also produce a new model for the energetic expenditure of walking, by modeling the leg as an inverted pendulum. Both approaches conclude that, with rigorous training, humans could perform normal locomotion at gravity no higher than 4g[subscript Earth].