An animal or man can be thought of as a series of fairly loosely connected sections. A leg consists of the thighbone connected to a ball and socket joint at the hip; the lower leg, with a hinge joint at the knee; and the foot which is fairly fl exibly jointed at the ankle. The arm is jointed from the shoulder in a similar way. So if a subject’s shoulders are jerked backward, his hands tend to follow only after his arms are pulled into line with the centers of gravity of his hands (Fig. A).