Adaptation to coriolis force perturbations of postural sway requires an asymmetric two-leg model

Abstract

In the companion paper (Bakshi A, DiZio P, Lackner JR. J Neurophysiol. In press, 2019), we reportedhow voluntary forward-backward sway in a rotating room generatedmedial-lateral Coriolis forces that initially deviated intended bodysway paths. Pure fore-aft sway was gradually restored over perrotationtrials, and a negative aftereffect occurred during postrotationsway. Force plate recordings showed that subjects learned to compensatefor the Coriolis forces by executing a bimodal torque, thedistribution of which was asymmetric across the two legs and ofopposite sign for forward vs. backward sway. To explain these results,we have developed an asymmetric, nonparallel-leg, inverted pendulummodel to characterize upright balance control in two dimensions.Fore-aft and medial-lateral sway amplitudes can be biomechanicallycoupled or independent. Biomechanical coupling occurs when Coriolisforces orthogonal to the direction of movement perturb sway aboutthe ankles. The model includes a mechanism for alternating engagement/disengagement of each leg and for asymmetric drive to theankles to achieve adaptation to Coriolis force-induced two-dimensionalsway. The model predicts the adaptive control underlying theadaptation of voluntary postural sway to Coriolis forces. A stabilityanalysis of the model generates parameter values that match thosemeasured experimentally, and the parameterized model simulationsreproduce the experimentally observed sway trajectories.