Many mathematical models of human movement have sought to represent as much as possible of the complexity of the human body but others, the subjects of this review, are extremely simple. Some treat the body as a point mass walking on rigid, massless legs or bouncing along on a spring. Others incorporate a few limb segments with appropriate masses, operated in some cases by a few muscles with realistic physiological properties. These simple models have been used to tackle questions such as these: why do we walk at low speeds but break into a run to go faster? Why do we change the length of our strides, and the patterns of force we exert on the ground, as we increase speed? Why do high jumpers run up more slowly than long jumpers and set down the take-off leg at a shallower angle? Why do we activate muscles sequentially, when throwing a ball? In every case the explanatory power of the model is enhanced by its simplicity.