Strategies of postural orientation and equilibrium in quadrupedal stance

Strategies for postural orientation and equilibrium were examined in cats. During quiet stance, changes in stance distance were accommodated by constraining trunk and intralimb geometry, while simultaneously changing the inclination of limb axes and ground reaction force (GRF) vectors. A cost function, based on the sum of squared joint torques, was found to be minimal at a stance distance naturally preferred by the cat. When stance was perturbed by translations of the support surface, equilibrium was maintained by parallel control of the GRF amplitude and direction. The amplitude was controlled by one set of mono- and biarticular muscles, whereas the direction was controlled by another distinct set of biarticulars. With spinal lesions, postural orientation and weight support in hindlimbs were maintained but horizontal stability was severely compromised, due to absence of equilibrium responses in hindlimb muscles. Thus, different roles of the spinal and supraspinal systems may be implied in the control of force amplitude and direction, respectively