A Unified Theory Regarding A/P and M/L Balance in Quiet Stance. Winter, David A., Fran[cedilla]cois Prince, Jim S. Frank, Corrie Powell, Karl F. Zabjek. Department of Kinesiology, University of Waterloo, Waterloo, Ontario, Canada N2L 3G1, Centre de recherche en g[acute]erontologie et g[acute]eriatrie, H[circumflex]opital Youville, Sherbrooke, Qu[acute]ebec, Canada J1H 4C4.
APStracts 2:037N, 1995.
SUMMARY AND CONCLUSIONS
Control of posture in quiet stance has been quantified by center of pressure (COP) changes in the anterior-posterior (A/P) and medial-lateral (M/L) directions from a single force platform. By recording from a single force platform researchers are unable to recognize two separate mechanisms that became evident when two force platforms are used. Depending on the stance position taken many combinations of an ankle mechanism and a hip (load/unload) mechanism are evident. In side-by-side stance A/P balance is totally under ankle (plantar/dorsiflexor) control, while M/L balance is under hip (abductor/adductor) control. In tandem stance the A/P balance is dominated by the hip mechanism with mixed and small or sometimes negligible contributions by the ankle plantar/dorsiflexors; for M/L balance the reverse is evident: ankle invertors/evertors dominate with mixed and small contribution from the hip load/unload mechanism. In an intermediate 45 degrees stance position both ankle and hip mechanisms contribute to the net balance control in totally different ways. In the M/L direction the two strategies reinforce while in the A/P direction the ankle mechanism must overcome and cancel most of the inappropriate contribution by the hip load/unload mechanism. A spatial plot of the separate mechanisms reveals the fact that the random looking (COP) scatter plot is nothing more than a spatial and temporal summation of two separate spatial plots. A straight line joining the individual centres of pressure under each foot is the load/unload line controlled by the hip mechanism. At right angles to this load/unload line in the side-by-side and tandem positions is the independent control line by the ankle muscles. In an intermediate standing position the separate control lines exist but now the ankle control is not orthogonal to the load/unload line; rather it acts at an angle of about 60 degrees . The direction of these ankle control and load/unload lines also allows us to pinpoint the muscle groups responsible at the ankle and hip in any of the stance positions. 

Received  1995; accepted in final form  1995.
APS Manuscript Number J.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1995 The American Physiological Society.
Published in APStracts on 23 December 95