There are three major joints of the lower limbs. The hip joint, which is a ball & socket joint, is designed to move in all directions. The knee joint is a hinge joint, designed only to bend and flex. The ankle joint is a saddle joint, where the ends of the two lower leg bones act like the legs of a rider on a saddle - the ankle allows dorsiflexion and plantarflexion as well as small movements which roll the ankle in and out (inversion & eversion). These three joints are meant to work together as we move such that no one joint needs to compensate for another's lack of mobility.