The purpose of locomotion is to transport the body through space. It requires the control of both propulsive and antigravitational forces with the capability to adapt to the changing surroundings. During locomotion, mechanisms within the body must simultaneously: control feet placement, speed and direction, maintain equi-
librium and negotiate obstacles. Gait is the precise sequence of leg and body movements that enables a legged system to move from one place to another. A general mathematical description of gait from a robotics perspective has been given by Sun (1974).