Once-Per-Step Control of Ankle Push-Off Work Improves Balance in a Three-Dimensional Simulation of Bipedal Walking

Individuals with lower limb amputation have a high fall risk, which could be partially due to a lack of stabilizing control in conventional prostheses. Inspired by walking robots, we hypothesized that modulating prosthetic ankle push-off could help improve amputee balance. We developed a three-dimensional walking model, found limit cycles at two speeds, and designed state-feedback controllers that made once-per-step adjustments to ankle push-off work, fore-aft and medial-lateral foot placement, and ankle roll resistance. To assess balance, we applied increasing levels of random changes in ground height and lateral impulses until the model fell down within 100 steps. Although foot placement is known to be important for balance, we found that push-off control was at least twice as effective at recovering from both disturbances at both speeds. Push-off work affected both fore-aft and mediolateral motions, leading to good controllability, and was particularly well suited to recovery from steps up or down. Our results suggest that discrete control of ankle push-off may be more important than previously thought, and may guide the design of robotic prostheses that improve balance.