Maintaining upright posture during perturbed standing in a motor-assisted hybrid neuroprosthesis with powered ankle joints: A feasibility and proof-of-concept study.

Introduction: Maintaining balance during perturbations is essential for the effectiveness of exoskeletal assistive devices in individuals with spinal cord injuries (SCI). We tested new ankle actuators in a muscles-first, motor-assisted hybrid neuroprosthesis (MAHNP) to evaluate their ability to maintain upright posture under various perturbations.

Methods: Participants with SCI (n = 2) performed standing balance tasks while wearing the MAHNP with and without ankle control, combined with electrical stimulation for one participant. A proportional, integral, derivative (PID) controller maintained 5° dorsiflexion based on input from an angle encoder. MAHNP's balance control mechanisms were evaluated by the center of pressure (CoP) excursion during unexpected perturbations for both participants, while hands-free standing, range-of-motion tasks, and functional reach tests were completed by one participant.

Results: Active control provided important functional benefits by improving forward reach by 4.3 cm compared to no control, and extending hands-free standing time by 28.3% compared to no control. Additionally, variance in mediolateral CoP excursion was reduced from approximately 57% with no control to around 50% with control.

Conclusion: These findings suggest that while PID-controlled ankle actuators perform well in quiet standing, further optimization may be required for tasks involving more dynamic movements and voluntary postural tasks.