A human postural regulation model with ankle and hip coordination

In this work, we present a computational model of a quietly standing human. The model is composed of a system of three rigid links with two joints, one at the ankle and one at the hip. This is consistent with considerable experimental data which indicates that humans keep their knee angle nearly constant when dealing with small perturbations to their posture. An optimal control problem is designed with a cost function in which the cost for errors in the center of pressure or of mass is quartic and the cost for control is quadratic. This performance measure tolerates larger state errors than the usual quadratic criterion and reduces the energy expended by the controller, a plausible goal for a neuronal controller. We have successfully solved the resulting constrained nonlinear optimal control problem. Simulations of the optimal control demonstrate better agreement with experimental studies of posture than other analytical models.