Robot-Aided Training of Propulsion: Effects of Torque Pulses Applied to the Hip and Knee Joint Under User-Driven Treadmill Control

Objective: to establish whether torque pulses applied by an exoskeleton to the hip and knee joint modulate propulsion mechanics and whether changes in propulsion mechanics are sustained after exposure to torque pulses under user-driven treadmill control. Methods: we applied twelve formulations of torque pulses consecutively over 300 strides to 22 healthy participants, and quantified the evolution of four outcome measures – gait speed (GS), hip extension (HE), trailing limb angle (TLA), normalized propulsive impulse (NPI) – before, during, and immediately after training. Results: Metrics of propulsion mechanics significantly changed both during and after training. Increases in HE during and after training were observed primarily in conjunction with hip/knee flexion pulses during early stance, or hip/knee extension during late stance. Increases in NPI during training were associated with hip/knee extension during early stance, or knee flexion during late stance. Knee flexion during early stance resulted in positive after-effects in NPI. Increases in GS were associated with the application of hip flexion pulses. Conditions exhibiting the largest positive changes in HE, and not NPI, during training resulted in increased GS after training. Analysis of the relationship between the effects measured during and after training suggests that after-effects primarily arise from retention of training effects, and that such retention is amplified compared to fixed-speed training. Conclusion and significance: Combination of exoskeleton training and user-driven treadmill control modulates propulsion mechanics both during and after training and can be considered for the formulation of propulsion-oriented methods for individuals with impairments in propulsion mechanics.