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.