Optimizing joint impedances to quickly reject an endpoint force perturbation in a cat hindlimb

Lower-limb joint impedance is governed by inter-muscular length, velocity, and force-dependent feedback. These feedback pathways regulate the stiffness and damping properties of the lower-limb by modulating muscle activation. The network of length, velocity, and force-dependent feedback pathways is interconnected, widely distributed, and not well- understood. Understanding the organization of this combined network will inform rehabilitation techniques for spinal cord injury. As a first step to understanding how these pathways are organized to reject perturbations, we developed a model of a cat hind limb and calculated the optimal distribution of homonymous length and velocity-dependent feedback to reject a force perturbation applied at the distal end of the limb during swing. The optimal solution resulted in the knee absorbing 80.9% of the energy from the perturbation, while the hip and ankle absorbed 18.1 and 1.1% of the energy, respectively.