Anatomy of motion.

Review of muscle function during the walking gait cycle suggests that once the body is put in motion, its own kinetic force tends to keep it moving. This progression, however, must be controlled and stabilized by muscles acting on the limb segments during weight bearing. Primary, accelerating muscle action is most important during swing phase when the iliopsoas and quadriceps femoris bring the limb forward to maintain continuity of the gait cycle. Once the limb is in stance phase, most muscles exert controlling forces on the limb by decelerating movement of the limb segments and stabilizing the joints.