Wearable robotic exoskeletons hold great promise for gait rehabilitation as portable, accessible tools. However, a better understanding of the potential for exoskeletons to elicit neural adaptation-a critical component of neurological gait rehabilitation-is needed. In this study, we investigated whether humans adapt to bilateral asymmetric stiffness perturbations applied by a hip exoskeleton, taking inspiration from the asymmetry augmentation strategies used in split-belt treadmill training. During walking, we applied torques about the hip joints to repel the thigh away from a neutral position on the left side and attract the thigh toward a neutral position on the right side. Six participants performed an adaptation walking trial on a treadmill while wearing the exoskeleton. The exoskeleton elicited time-varying changes and aftereffects in step length and propulsive/braking ground reaction forces, indicating behavioral signatures of neural adaptation. These responses resemble typical responses to split-belt treadmill training, suggesting that the proposed intervention with a robotic hip exoskeleton may be an effective approach to (re)training symmetric gait.

Wearable robotic exoskeletons show promise in their ability to provide gait assistance and rehabilitation in real-world contexts. However, a better understanding is needed of how exoskeletons contribute to neural adaptation in locomotion, a critical component of neurological gait rehabilitation. We tested whether unilateral perturbations elicit neural adaptation in healthy participants using a novel robotic hip exoskeleton, taking inspiration from asymmetry augmentation strategies used in split-belt treadmill training. We found that applying a virtual stiffness parallel to the hip joint on one side elicited changes in hip range of motion and step length, and that these changes were time varying, indicating an adaptation response. However, participants converged on asymmetric hip ranges of motion and step lengths both with and without applied stiffness from the exoskeleton. These results suggest that while adaptation appears to have occurred, it was not solely driven by the nervous system reducing gait asymmetry. Our findings indicate that applying mechanical impedance asymmetrically to the joints may be an effective gait training and rehabilitation approach, as well as a method to elicit a novel adaptation response to further study neuromotor control of locomotion.

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.