Walking relies on a repetitious pattern of limb motion, under selective muscle control, to accomplish four functions. Two of these functions, progression and weight-bearing stability, are essential. Two supplementary functions, shock absorption and energy conservation, improve the quality of walking. To accomplish these functions, the two limbs, in reciprocal fashion, participate in four action patterns. The action patterns are foot support, motion at each joint (foot, ankle, knee, hip, pelvis, and trunk), floor reaction forces, and muscle control. Each is a sequence of events. Normal gait represents selective integration of the appropriate elements of each action pattern in a sequential fashion. The overlap of multiple events occurring in the gait cycle creates eight patterns of limb function. The area of foot support continually changes during each stance period as body weight rolls forward.