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 rej ection in response to anterior disturbances, a mostly linear relationship b etween the ankle torque and ankle angle was observed, resulting in a consta nt stiffness at the ankles. However, when the ankle torque saturation was r eached, a combined ankle-trunk postural strategy was utilized mainly in res ponse to posterior perturbations due to the properties of the base-of-suppo rt. This caused a nonlinear scaling of ankle responses, thereby increasing variability of ankle stiffness. Distinctions in the ankle responses were al so observed for different initial standing postures. The anterior initial s tance generally increases the overall postural stability and renders the ut ilization of ankle strategy feasible, even for the rejection of posterior d isturbances. Therefore, a linear,torque-angle relationship at the ankles wa s observed for the anterior initial stance, regardless of the perturbation parameters. (C) 2000 Elsevier Science B.V. All rights reserved.