Gravity-driven horizontal locomotion: theory and experiment

Gravity is usually considered not to contribute net energy to horizontal locomotion, the force of gravity being at right angles to the velocity vector. However, when walking or running, humans essentially rotate around the support foot while falling forward during the support phase of the stride. The body thereby acquires angular momentum and rotational energy that are entirely due to the force of gravity, which may be coupled into the forward motion as a source of propulsion upon landing on the other foot. A theory is developed to determine the magnitude and nature of these effects of gravity, showing that more than 10% of the energy needed for running can be obtained from the field of gravity. Likewise, at a particular optimum velocity, walking may become entirely driven by gravity-induced angular momentum without any muscular effort. Experiments with athletes running or walking on a treadmill—after appropriate training—are consistent with the theory: angular momentum-rich locomotion techniques would have applications in sports, competition running and the mechanics of walking, and in the making of leg prostheses and exoskeletons, and might help to explain how humans can outrun larger prey animals to exhaustion during hours of hunting pursuits.