In human bipedal stance,
the narrow support surface provided by the foot area and
height of the body with respect to the ground make
for a much smaller safety margin than in quadrupeds.
It has been established previously that after stance disturbances,
one of the strategies used, namely the hip
strategy, consists of flexing or extending the hip, in order
to keep the CG within the stability limits The
same change in body geometry occurs when a voluntary
movement of the upper trunk is performed; this would
entail a movement of the CG in the same direction if no
corrective processes intervened. Opposite displacements
of the lower segments then occur however, which results
in the maintenance of CG within the support area.
This has been termed ‘synergy’ by Babinski but a
more appropriate name might be ‘kinematic strategy’, in
line with the hip strategy known to counteract imposed
disturbances. These findings indicate that in humans, the
body geometry changes in order to regulate the CG position.
The question then arises as to how the CG is regulated,