Abstract
The study examines postural activity of a constrained subject in response to various disturbances in the sagittal plane. Three different initial standing postures were examined. Each response to a perturbation was divided into three consecutive phases according to the intensity of the muscular activity. The relation of the ankle joint torque versus the ankle joint angle was studied. A linear relationship, resulting in constant ankle stiffness, was observed in each phase of the response. Only negligible differences in the stiffness values were observed among different phases. The results indicate an ankle stiffness value of 10 N m/° for the majority of initial stance postures and perturbation intensities.