Determinants of postural orientation in quadrupedal stance.

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.