Frequency-Dependent Force Direction Elucidates Neural Control of Balance

Abstract

Background: Maintaining upright posture is an unstable task that requires control of translational and rotational motions. Humans use foot-ground interaction force, characterized by point of application, magnitude, and direction to manage body accelerations. Previous work identified a point of intersection of the foot-ground interaction force vectors that exhibited consistent frequency-dependent behavior. Methods: To test whether this frequency-dependent behavior provided a distinctive signature of neural control or was a necessary consequence of biomechanics, this study simulated quiet standing and compared the results with human subject data. If a standing human was modeled as a single inverted pendulum, no controller could reproduce the experimentally observed frequency-dependence of the intersection point height. The simplest competent model that approximated a standing human was a double inverted pendulum with torque-actuated ankle and hip joints. It was stabilized by a linear feedback controller based on position and velocity errors of each joint. Results: When the relative cost between state deviation and control effort was varied, the frequency at which the intersection point crossed the center of mass position shifted. A similar effect was obtained by varying the relative cost between the ankle and hip control effort. The relative strength of ankle and hip actuation noise added to the simulated system affected the intersection point height at high frequencies. Conclusions: As a range of controller parameter sets could stabilize this model and produce the observed change in the vertical position of the intersection point with increasing frequency, the decrease in intersection point height appears to reflect a biomechanical constraint and not a consequence of control. Among the several controller parameter sets considered, that which best reproduced the human experimental results used minimal control effort and more ankle torque than hip torque. This suggests that the neural strategy employed by human subjects to maintain quiet standing balance engages at least two degrees of freedom and is best described by minimal control eort and emphasizing ankle torque.

Mechanical Effects of Canes on Postural Control: Beyond Perceptual Information

“…Further, recent work went beyond analyzing the point of application of the force vector, and examined the orientation of the ground reaction force with respect to the center of mass. This analysis revealed interesting information about the relative role of biomechanics and control (2,26). Applying these methods to the more challenging task of standing on a narrow beam with canes could provide further information about the strategy adopted by the controller when using canes.…”