UNIFIED THEORY REGARDING A P AND M/L BALANCE IN QUIET STANCE/

1. Control of posture in quiet stance has been quantified by center of pressure (COP) changes in the anterior-posterior (A/P) and medial-lat eral (M/L) directions from a single force platform. Recording from a s ingle force platform, researchers are unable to recognize two separate mechanisms that become evident when mio force platforms are used. Dep ending on the stance position taken, many combinations of an ankle mec hanism and a hip (load/unload) mechanism are evident. In side-by-side stance, A/P balance is totally under ankle (plantar/dorsiflexor) contr ol, whereas M/L balance is under hip (abductor/adductor) control. In t andem stance, the A/P balance is dominated by the hip mechanism, with mixed and small or sometimes negligible contributions by the ankle pla ntar/dorsiflexors: for M/L balance, the reverse is evident; ankle inve rtors/evertors dominate, with mixed and small contribution from the hi p load/unload mechanism. In an intermediate 45 degrees stance position ,both ankle and hip mechanisms contribute to the net balance control i n totally different ways. In the M/L direction the two strategies rein force, whereas in the AIP direction the ankle mechanism must overcome and cancel most of the inappropriate contribution by the hip load/unlo ad mechanism. A spatial plot of the separate mechanisms reveals the fa ct that the random-looking COP scatter plot is nothing more than a spa tial and temporal summation of two separate spatial plots. A straight line joining the individual COPs under each foot is the load/unload Li ne controlled by the hip mechanism. At right angles to this load/unloa d line in the side-by-side and tandem positions is the independent con trol line by the ankle muscles. In an intermediate standing position, the separate control lines exist, but now the ankle control is not ort hogonal to the load/unload line; rather, it acts at an angle of simila r to 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.