Biomechanics of the foot and ankle.

The human foot evolved to provide: (1) mechanisms to limit the excursion of the center of mass during ambulation and thereby minimize the expenditure of energy; (2) a base of support of sufficien dimensions for the stability necessary to maintain the upright position without excessive muscular activity; (3) mechanisms for flexibility to absorb the shock of the body weight and for accommodation to uneven terrain; (4) rigidity of the foot when it acts as a lever in the push-off period of stance.