Changes in a postural strategy with inter-paw distance.

1. The purpose of this study was to examine the effect of changing initial stance conditions on the postural response of the cat to horizontal plane translations of the support surface. Cats were trained to stand, unrestrained, on a moveable force platform. The platform was translated linearly in each of 16 directions in the horizontal plane, with a ramp-and-hold displacement. The animal's response was quantified in terms of the forces exerted at the ground. The trajectory of the center of mass (CoM) was computed from the forces. 2. Stance length was varied along the longitudinal (sagittal) axis by adjusting the distance between the forepaw and hindpaw force plates. Translation perturbations of the platform were recorded at stance distances varying from 66 to 110% of the preferred stance distance. 3. Changing stance distance had a significant effect on the amplitude and direction of the active forces exerted by the cat both during quiet stance and during the response to platform translation. At long stance distances, each limb exerted a force outward, along the diagonals during quiet stance. The response to translation was characterized by an invariance in the direction of force exerted against the ground, a strategy that was described previously. At short stance distances, quiet stance forces were more laterally directed. The force constraint strategy was usually not observed for the response to translation. Nevertheless, the cats were equally effective at all stance distances in restoring the position of the center of mass after translation of the support surface. 4. There was no discrete boundary between the presence and absence of the force constraint, suggesting that the strategy for exerting forces against the support surface is characterized by a continuum of response, from a bimodal, or anisotropic distribution of force vectors on the one extreme, to a uniform, or isotropic distribution on the other. Arguments are developed to suggest that the force constraint strategy may be useful in stabilizing the vertebral column during the response to platform translation, to allow linear translation of the CoM rather than bending of the trunk.