A Feasibility Study of Piecewise Phase Variable Based on Variable Toe-Off for the Powered Prosthesis Control: A Case Study

Achieving stable walking and proper assistance in prosthesis control requires synchronized control, treating the user and the prosthesis as a coupled system. Furthermore, speed adaptability is essential for controlling the prosthesis at different walking speeds. One approach involves using a phase variable to estimate the user's gait phase and synchronize prosthesis control accordingly. However, the current phase variable (i.e., PV) fails to reflect variable toe-off timing at different speeds, despite individuals having different toe-off timings per walking speed. To address this issue, we propose a piecewise phase variable (i.e., PW-PV) that accounts for different toe-off timings while estimating the user's gait phase at various speeds. We conducted a treadmill walking experiment with two participants (one healthy and one amputee) using a custom-built powered prosthesis to validate the PW-PV's feasibility. We collected and analyzed joint kinematics, kinetics, and ground reaction force data during the experiment. The PW-PV implementation resulted in faster load transfer and a more natural rollover for both participants during walking. This allowed healthy and amputee participants to experience longer push-off durations of 10.6% and 15.2%, respectively, and greater ankle push-off work of 7.3% and 16.9%. Furthermore, with the PW-PV, the amputee participant demonstrated higher vertical ground reaction forces of 5.4% and 4.7% on her prosthesis side leg during load acceptance and push-off periods, potentially suggesting increased confidence in using the prosthesis. We anticipate that by using the proposed phase variable, we will be able to provide more appropriate and timely assistance to individuals at variable walking speeds.