The equipment was devised for the projection into the base of support of concurrent shifts of the center of gravity in the 2 cardinal vertical orientation planes of a standing subject. Repeated observations were made on 10 young adult women. The center of weight shifts incessantly, standing being in reality movement upon a stationary base. The asynchronous rotation of motor units which gives to postural contraction its unique indefati-gability may be accounted for by the resultant shifting stimulation of stretch afferents.