CHANGES IN A POSTURAL STRATEGY WITH INTER-PAW DISTANCE

1. The purpose of this study was to examine the effect of changing ini tial stance conditions on the postural response of the cat to horizont al plane translations of the support surface. Cats were trained to sta nd, unrestrained, on a moveable force platform. The platform was trans lated 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 cente r of mass (CoM) was computed from the forces. 2. Stance length was var ied along the longitudinal (sagittal) axis by adjusting the distance b etween 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 for ces exerted by the cat both during quiet stance and during the respons e to platform translation. At long stance distances, each limb exerted a force outward, along the diagonals during quiet stance. The respons e to translation was characterized by an invariance in the direction o f force exerted against the ground, a strategy that was described prev iously. At short stance distances, quiet stance forces were more later ally directed. The force constraint strategy was usually not observed for the response to translation. Nevertheless, the cats were equally e ffective at all stance distances in restoring the position of the cent er of mass after translation of the support surface. 4. There was no d iscrete boundary between the presence and absence of the force constra int, suggesting that the strategy for exerting forces against the supp ort surface is characterized by a continuum of response, from a bimoda l, or anisotropic distribution of force vectors on the one extreme, to a uniform, or isotropic distribution on the other. Arguments are deve loped to suggest that the force constraint strategy may be useful in s tabilizing the vertebral column during the response to platform transl ation, to allow linear translation of the CoM rather than bending of t he trunk.