Once per step ankle push-off work control is an effective method to improve balance.

Individuals with lower-limb amputation experience increased fall risk, which could be partially due to a lack of active control in conventional prostheses. Inspired by stabilization strategies from planar walking robots, we hypothesized that modulating prosthetic ankle push-off could help improve amputee balance. We developed a simple three-dimensional walking model 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 until the model fell down within one hundred steps. Although foot placement is known to be important to balance, we found push-off control to be more effective in this case. With push-off control, the model tolerated unexpected changes in ground height of 7.8% of leg length on each step, while foot placement and ankle roll resistance allowed changes of only 1.5% and 0.8%, respectively. Push-off affected both fore-aft and medial-lateral motions, such that commanding more work during unexpected steps up or down tended to correct errors in both directions simultaneously. This result suggests 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.


Figure: Push-off affects frontal plane dynamics. 

The force generated by push-off (F), usually described in sagittal plane, can also affect frontal plane motions. With finite medial-lateral distance between the foot and the center of mass, the combined effects of push-off force and body weight lead to medial-lateral force at the foot and a medial-lateral component of body acceleration


Animation: Ankle push-off work is an effective method to restore balance from random disturbances in ground heights.

Talk: Dynamic walking 2013


         M. Kim and S.H. Collins, Once-per-step control of ankle push-off work improves balance in a three-dimensional simulation of bipedal walking, in review.

         M. Kim, S.H. Collins, Stabilization of a three-dimensional limit cycle walking model through step-to-step ankle control, In Proceedings of International Conference on Rehabilitation Robotics, 1-6, 2013. pdf