Yes, for a run he must push off the ground with his back leg, then extend his other leg in front of him to catch himself. His torso is moving forward at a constant velocity, so when it hits the ground then the bodies weight rests on it forcing it to bend at the knee, then the body passes over that leg, and the person uses the bend in the knee and the ankle to push off the ground once again. The difference between a run and a walk is that in a walk, you are simply leaning forward, and constantly catching yourself with alternating legs. In a run, you are jumping forward and catching yourself with alternating legs.