Journal of Neuroscience, Vol 15, 1121-1131, Copyright © 1995 by Society for Neuroscience
Determinants of postural orientation in quadrupedal stance
J Fung and JM Macpherson
R.S. Dow Neurological Sciences Institute, Portland, Oregon 97209.
The purpose of this study was to investigate the determinants of postural
orientation by examining stance kinematics and kinetics at various interpaw
distances. Four adult cats were trained to stand, unrestrained, on a force
platform. Three-dimensional ground reaction forces and kinematics, as well
as EMG activities, were recorded during stance at five different
anteroposterior (AP) distances and two widths. Stance distance was varied
by changing the distance between the four force plates mounted on the
platform. All cats used a strategy of maintaining a constant horizontal
orientation of the trunk and levering the limbs at the girdles while
maintaining constant intralimb geometry to accommodate the changes in
stance distance. The direction of the ground reaction forces covaried with
the limb axes. As a result, the joint torques were conserved in the
forelimb, and varied within small ranges in the hind. Increased tonic
activity in the extensors of the back, the hip, and the ankle was observed
at shorter distances while increased knee extensor activity was observed at
longer distances. A cost function, defined as the sum of squared 3-D joint
torques, was minimal for the hindlimb at a stance distance which
corresponded to the preferred distance naturally assumed by each cat on the
floor. Thus, in the maintenance of stance posture, trunk orientation and
intralimb geometry is constrained, the goal of which is to minimize
muscular effort or energy expenditure.
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