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

To achieve stable walking and provide proper assistance, it is crucial to have synchronized control of the prosthesis, treating the user and the prosthesis as a coupled system. Additionally, speed adaptability is important for controlling the prosthesis at different walking speeds. One approach to achieving this is by using a phase variable to estimate the user's gait phase and control the prosthesis in synchrony with the user. However, the current phase variable (i.e., PV) cannot reflect variable toe-off timing at different speeds, although individuals have different toe-off timings per walking speed. To address this issue, we propose a piecewise phase variable (i.e., PW-PV) that can be adjusted for different toe-off timings while estimating the user's gait phase at various walking speeds. As a case study, we conducted a treadmill walking experiment with two participants (i.e., one healthy and one amputee) using a custom-built powered prosthesis. We collected and analyzed joint kinematics, kinetics, and ground reaction force data to validate the feasibility of the PW-PV. The use of the PW-PV resulted in both participants experiencing faster load transfer and a more natural rollover while walking. This allowed healthy and amputee participants to walk with 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.