Maintaining upright posture during perturbed standing in a motor-assisted hybrid neuroprosthesis with powered ankle...

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.

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.

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.

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.