Muscle-reflex model of human locomotion entrains to mechanical perturbations

Prior experiments have shown that human gait synchronizes to periodic torque pulses applied about the hip and ankle joints by robotic exoskeletons. Importantly, entrainment occurred even when the pulse period differed slightly from the user's preferred stride period, making it a viable approach to increase gait speed. As gait speed is an important outcome of gait therapy, gait entrainment to mechanical perturbations may serve as a promising new method of robot-aided therapy. Still, an understanding of the underlying neuromechanical processes that give rise to gait entrainment is needed to fully evaluate its therapeutic potential. To gain such insight, the goal of this paper was to evaluate whether an existing neuromechanical model of human locomotion exhibited entrainment behavior similar to that observed in the prior human experiments. Simulation results showed that the model entrained to pulses applied at both the ankle and hip joints. The convergence of relative phase between model gait and hip perturbations was similar to that observed with the human gait, but differed slightly for ankle perturbations. Thus, models that can more accurately describe neuromechanical interactions between human gait and robotic exoskeletons are still needed. Nevertheless, the simulation results support the notion that the limit-cycle behavior observed during locomotion does not require supra-spinal control or a self-sustaining oscillatory neural network, which has important implications for improving gait therapy.