In addition to the stabilization of the CG position, the
orientation of several body segments has also been reported
to be regulated simultaneously during stance,
movement or locomotion. For example, the head axis
is often stabilized with respect to the vertical during locomotion
[19,20]. This stabilization provides a reference
value aligned with the vertical axis, used for monitor-ing, via visual and vestibular inputs, the head and body
movements with respect to the environment. Head- and
trunk-centered reference Ii-ames are also used for target
location and movement trajectory planning (see [21]).
The trunk axis is actively stabilized during locomotion
in both the frontal and sagittal planes, due to the action
of the hip muscles . The trunk stabilization
in the frontal plane is increased in labyrinthine defective
patients . The trunk axis also remains vertically oriented
during leg raising, especially in dancers [25]. As
the trunk axis serves as an egocentric reference frame
for calculating leg position , vertical orientation of
the trunk may be used for direct calculation of the leg
position with respect to space.