This study confirms the indirect knee-stabilizing function of the ankle plantar flexors in walking on the level. By combined simultaneous electromyograms and motion pictures of gait the periods of activity of the ankle plantar flexors and of increasinig knee extension and dorsiflexion of the foot were shown to correspond. Only at the end of the period of ankle plantar-flexor action did plantar flexion of the foot occur. Knee extension took place after the cessation of activity of the quadriceps muscle and did not appear to be produced by quadriceps action. An alternate hypothesis is offered instead of the theory that relates knee extension in the stance phase to movement of the center of gravity in front of the line of support. This hypothesis is that knee extension during the stance phase, in ordinary walking on the level, is brought about by the force of the ankle plantar flexors resisting the dorsiflexion of the ankle, this dorsiflexion being produced by the resultant of the extrinsic forces (kinetic forces, gravity, and the floor reaction). The resultant of the extrinsic forces is greater than the intrinsic force, as manifested by the increasing dorsiflexion of the foot which occurs up until heel-off begins. The function of the calf muscles in toe-off has been widely discussed by other authors and, therefore, is not emphasized in this report. The restraining function of the ankle Plantar flexors in decelerating forward rotation of the tibia on the talus is the key to their stabilizing action.