The human body is a complex structure, but our understanding of its movements is greately enhanced by extremely simple mathematical models. A model of walking shows why we have to break into a run at speeds above 3 meters per second, and why the critical speed is lower for children and on the moon. A model of running jumps show why long jumpers run up at a fast sprinting speed, but high jumpers run up much more slowly. Finally, a model od standing jumps explains why athletes can jump higher by using a countermovement than from a static squatting position. The body is represented in these models as a small number of rigid segments connected by hinge joints, powered by muscles with realistic physiological properties.