A Biomechanical Model That Confirms Human Ankle Angle Changes Seen During Short Anterior Perturbations of a Standing Platform

Humans sway. With bipedal stance on a fixed base of support, this sway causes changes in the location of the Center of Pressure as it is projected onto this base of support. Translating the base itself also causes changes in the Center of Pressure. In this study, we have developed a two-dimensional biomechanical model that uses changes in the Anterior-Posterior Center of Pressure (APCOP) to track ankle angle changes arising from 16 mm anterior displacement perturbations of a platform on which a subject stands. The model uses the total torque generated at the ankle joint as one of the inputs, and calculates it assuming a PID controller. The necessary stiffness and damping is provided by the P and D components of the controller. This feedback system generates ankle torque based on the angular position of the center of mass (COM) with respect to vertical line passing through the ankle joint. This study also assumes that the internal components of the net torque are controller torque and sway pattern-generating torque. The final inputs to the model are the platform acceleration and some anthropometric terms. This model of postural sway dynamics predicts sway angle and the trajectory of the center of mass; and points out the relationships among the biomechanical variables like ankle angle, torque, center of pressure, and center of mass.