From the flexed and symmetrical posture of the supine full term newborn, an asymmetry develops in which the upper extremity on the side to which the head is rotated extends, while that on the side of the occiput is flexed , thus being described as a ‘fencing position’ (see figure below). Also, the leg on the side of the face tends to be extended, while the other leg is flexed. If the posture is imposed by passively turning the head, it appears earlier than its spontaneous counterpart. Both reach a peak during the second to fourth months, and then gradually disappear as a more symmetrical posture is regained. The mechanisms are poorly understood, but it is thought that the posture arises from the increasing influence of extensor over flexor neurons.
