Can muscle stiffness alone stabilize upright standing?

A stiffness control model for the stabilization of sway has been proposed r ecently. This paper discusses two inadequacies of the model: modeling and e mpiric consistency. First, We show that the in-phase relation between the t rajectories of the center of pressure and the center of mass is determined by physics, not by control patterns. Second, we show that physiological val ues of stiffness of the ankle muscles are insufficient to stabilize the bod y "inverted pendulum." The evidence of active mechanisms of sway stabilizat ion is reviewed, pointing out the potentially crucial role of foot skin and muscle receptors.