1. It has previously been demonstrated that a set of geometric and kinetic parameters are invariant in cats standing at their preferred interfoot distance and weight distribution. Thus the length and the angle of orientation relative to the vertical of each limb axis remain approximately constant when the supporting platform is tilted in the sagittal plane. The direction of the tangential contact forces is similarly constrained in response to horizontal translations. The main aim of the present study is to assess whether or not the control of limb position 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 conditions expressly designed to increase the range of postural variability. We considered that if the specification of limb position is a mere byproduct of the neural control of contact forces (or vice versa), geometric and kinetic parameters would covary interdependently. If instead limb position and contact forces are controlled in parallel and independently of each other, they will tend to follow different laws of variation. 2. Limb position and contact forces were measured in intact cats standing freely on a support platform. In a first series of experiments the pitch angle of the platform was randomly changed, as were the interfoot distance and head orientation. In another series of experiments cats were tilted in the presence of an external load tending to shift the weight distribution. The same load was applied in two different manners: 1) it made contact with a very limited surface of the body, and 2) it was attached by means of a long vest that made contact with most of the trunk and produced abnormal somesthesic cues to the body. 3. The range of different experimental conditions resulted in substantial trial-to-trial variations of the length and orientation of the axis of the limbs, as well as variations of the magnitude and orientation of the net contact forces. We found that the changes of the orientation of the contact force vector are uncorrelated with the corresponding changes of limb orientation, thus providing a first line of evidence in favor of the existence of a separate neural control of geometric and kinetic parameters. 4. Another line of evidence is provided by the specific form of the laws of variation of geometric parameters and tangential forces in different animals. Under normal (unloaded) conditions the values of the limb joint angles tend to covary linearly. The best-fitting planar regression of the angular values requires an absolute reference to the vertical. The orientation of the plane is highly consistent in all cats and it is essentially identical at the forelimbs and at the hindlimbs, despite their large biomechanical differences. 5. The normal contact forces, as well as the strut component of the tangential contact forces, are completely determined by the assigned postural geometry. The lever component (exerted by proximal muscles) of the tangential contact forces is instead controlled independently of limb geometry. This force is accurately partitioned between forelimbs and hindlimbs in all cats. The specific proportion of force partitioning, however, is idiosyncratic to each animal and highly variable among animals. 6. The existence of a neural control of limb geometry independent of the control of limb kinetics is borne out also from the experiments involving the application of a load that shifted the center of mass of the body. When the load made contact with a limited surface of the body, we found consistently that limb geometry (mean length and orientation of limb axis, and planar covariation of joint angles) was preserved unaltered, whereas kinetic parameters (magnitude of the contact forces and joint torques) were severely affected. 7. Limb geometry was altered, however, when the load was attached by means of a long vest that made contact with most of the trunk, resulting in anomalous somesthesic stimuli. Cats did not maintain the limbs vertical as under normal conditions, but rotated them almost in parallel with the rotation of the table. Moreover, the orientation of the regression plane of limb joint angles was generally tilted. In contrast with the orientation of the limb axis, the orientation of the contact forces did not change relative to the control, thus providing another instance of independent control of limb geometry and contact forces.
|Number of pages||20|
|Journal||Journal of Neurophysiology|
|Publication status||Published - 1994|
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