 Walking gaits are all characterized by a "vaulting" movement of the body over the legs, frequently described as an inverted pendulum (displaying fluctuations in kinetic and potential energy which are perfectly out of phase).In running, the kinetic and potential energy fluctuate in-phase, and the energy change is passed on to muscles, bones, tendons and ligaments acting as springs (thus it is described by the spring-mass model).Speed generally governs gait selection, with quadrupedal mammals moving from a walk to a run to a gallop as speed increases.