INDEPENDENT CONTROL OF LIMB POSITION AND CONTACT FORCES IN CAT POSTURE

1. It has previously been demonstrated that a set of geometric and kin etic parameters are invariant in cats standing at their preferred inte rfoot distance and weight distribution. Thus the length and the angle of orientation relative to the vertical of each limb axis remain appro ximately constant when the supporting platform is tilted in the sagitt al plane. The direction of the tangential contact forces is similarly constrained in response to horizontal translations. The main aim of th e present study is to assess whether or not the control of Limb positi on is independent of the control of the contact forces at the feet. To this end we have examined cat posture under a number of different con ditions expressly designed to increase the range of postural variabili ty. We considered that if the specification of limb position is a mere byproduct of the neural control of contact forces (or vice versa), ge ometric and kinetic parameters would covary interdependently. If inste ad limb position and contact forces are controlled in parallel and ind ependently of each other, they will tend to follow different laws of v ariation. 2. Limb position and contact forces were measured in intact cats standing freely on a support platform. In a first series of exper iments the pitch angle of the platform was randomly changed, as were t he interfoot distance and head orientation. In another series of exper iments cats were tilted in the presence of an external load tending to shift the weight distribution. The same load was applied in two diffe rent manners: 1) it made contact with a very limited surface of the bo dy, and 2) it was attached by means of a long vest that made contact w ith most of the trunk and produced abnormal somesthesic cues to the bo dy. 3. The range of different experimental conditions resulted in subs tantial trial-to-trial variations of the length and orientation of the axis of the limbs, as well as variations of the magnitude and orienta tion of the net contact forces. We found that the changes of the orien tation of the contact force vector are uncorrelated with the correspon ding changes of limb orientation, thus providing a first line of evide nce in favor of the existence of a separate neural control of geometri c and kinetic parameters. 4. Another line of evidence is provided by t he specific form of the laws of variation of geometric parameters and tangential forces in different animals. Under normal (unloaded) condit ions the values of the limb joint angles tend to covary linearly. The best-fitting planar regression of the angular values requires an absol ute reference to the vertical. The orientation of the plane is highly consistent in all cats and it is essentially identical at the forelimb s and at the hindlimbs, despite their large biomechanical differences. 5. The normal contact forces, as well as the strut component of the t angential contact forces, are completely determined by the assigned po stural geometry. The lever component (exerted by proximal muscles) of the tangential contact forces is instead controlled independently of l imb geometry. This force is accurately partitioned between forelimbs a nd hindlimbs in all cats. The specific proportion of force partitionin g, however, is idiosyncratic to each animal and highly variable among animals. 6. The existence of a neural control of limb geometry indepen dent of the control of limb kinetics is borne out also from the experi ments involving the application of a load that shifted the center of m ass of the body. When the load made contact with a limited surface of the body, we found consistently that limb geometry(mean length and ori entation of limb axis, and planar covariation of joint angles) was pre served unaltered, whereas kinetic parameters ( magnitude of the contac t forces and joint torques) were severely affected. 7. Limb geometry w as altered, however, when the load was attached by means of a long ves t that made contact with most of the trunk, resulting in anomalous som esthesic stimuli. Cats did not maintain the limbs vertical as under no rmal conditions, but rotated them almost in parallel with the rotation of the table. Moreover, the orientation of the regression plane of li mb joint angles was generally tilted. In contrast with the orientation of the limb axis, the orientation of the contact forces did not chang e relative to the control, thus providing another instance of independ ent control of limb geometry and contact forces.