A nonlinear optimal human postural regulator

A postural control model for a quietly standing human is proposed. The musculoskeletal dynamics of the human is modeled as a triple inverted pendulum in the sagittal plan, including ankle, knee and hip joints. A nonlinear optimal control problem is defined to study the postural sway. It has a performance measure with quadratic terms in the controls and a quartic term in either the center of pressure (COP) or the horizontal projection of the center of mass (COM). This objective function provides a trade-off between the allowed deviations from the nominal value and the neuromuscular energy required to correct for these deviations. By using the Model Predictive Control (MPC) technique, the discrete-time approximation to each of these problems is converted into a nonlinear programming problem and then solved. The solution gives a control scheme that demonstrates qualitative agreement to the main features of the joint kinematics and coordination observed experimentally.