Postural activity of constrained subject in response to disturbance in sagittal plane.

This study examines the postural activity of partially constrained subjects for three different initial standing postures in response to disturbances in the sagittal plane. When the ankle strategy suffices for disturbance rejection in response to anterior disturbances, a mostly linear relationship between the ankle torque and ankle angle was observed, resulting in a constant stiffness at the ankles. However, when the ankle torque saturation was reached, a combined ankle-trunk postural strategy was utilized mainly in response to posterior perturbations due to the properties of the base-of-support. This caused a nonlinear scaling of ankle responses, thereby increasing variability of ankle stiffness. Distinctions in the ankle responses were also observed for different initial standing postures. The anterior initial stance generally increases the overall postural stability and renders the utilization of ankle strategy feasible, even for the rejection of posterior disturbances. Therefore, a linear torque-angle relationship at the ankles was observed for the anterior initial stance, regardless of the perturbation parameters.