Single or cyclical voluntary movements of flexion and extension of the hand, performed in a parasagittal plane, are immediately and naturally coupled with the same movements of the foot only if the extremities follow simultaneously the same direction. Instead, great care and attention are required to move the two segments in opposite directions, an association which tends to reverse spontaneously to the 'easy' pattern. This rule is followed independently of the muscles employed, since it holds both when the hand is prone and when it is supine. The same principle also applies to many other couples of voluntary movements of the ipsilateral limbs or of different segments in the same limb.