Assist-as-needed Control for FES in Foot Drop Management

TL;DR

Foot drop management with FES often relies on open-loop control with fixed stimulation, risking fatigue or insufficient dorsiflexion. This work introduces a toe clearance–driven closed-loop FES controller that adjusts stimulation in real time to ensure sufficient clearance. In experiments with healthy participants and induced foot drop across varying speeds and inclines, the closed-loop controller achieved comparable toe clearance to open-loop while delivering ~$34\%$ less TA stimulation, suggesting improved comfort and reduced fatigue. The results indicate potential for more comfortable, longer-term FES use in daily life, with future work focusing on robust toe-clearance estimation in real-world settings and patient trials.

Abstract

Foot drop is commonly managed using Functional Electrical Stimulation (FES), typically delivered via open-loop controllers with fixed stimulation intensities. While users may manually adjust the intensity through external controls, this approach risks overstimulation, leading to muscle fatigue and discomfort, or understimulation, which compromises dorsiflexion and increases fall risk. In this study, we propose a novel closed-loop FES controller that dynamically adjusts the stimulation intensity based on real-time toe clearance, providing "assistance as needed". We evaluate this system by inducing foot drop in healthy participants and comparing the effects of the closed-loop controller with a traditional open-loop controller across various walking conditions, including different speeds and surface inclinations. Kinematic data reveal that our closed-loop controller maintains adequate toe clearance without significantly affecting the joint angles of the hips, the knees, and the ankles, and while using significantly lower stimulation intensities compared to the open-loop controller. These findings suggest that the proposed method not only matches the effectiveness of existing systems but also offers the potential for reduced muscle fatigue and improved long-term user comfort and adherence.

Figures (9)

• Figure 1: Experimental setup including the calibration of the FES intensity for each participant for both the plantarflexor and dorsiflexor muscles (left), and marker-based motion capture using Vicon and the instrumented treadmill M-GAIT (middle), for the different conditions shown on the right.
• Figure 2: (a) Image of the shoe with reflective markers. (b) 3D scan of the shoe including reflective markers. (c) Point cloud of the shoe where black dots indicate the base of the shoe and red dots indicate the location of the centre of the reflective markers. (d) Toe clearance estimation based on the recorded position of the reflective markers (shown in red), the anterior-half points of the base of the shoe (shown in orange) and an estimation of the ground (shown as a dashed line).
• Figure 3: Mean toe clearance ($\pm$SD) of all recorded cycles from all subjects under the different conditions including toe clearance during slow walk (top row) and fast walk (bottom row), toe clearance during decline (left column), level walking (middle column) and incline (right column), and toe clearance during FES OFF (in green), FES FD (in red), FES OL (in orange) and FES CL conditions (in blue). For clarity, the variance of the recorded toe clearance is also provided in Table \ref{['tab:toe_clearance_variability']}. Vertical bars ($|$) indicate points in the gait cycle where mean differences between conditions are statistically significant (p$<$0.05).
• Figure 4: Mean ankle angle during the swing phase for all subjects under the different conditions including toe clearance during slow walk (top row) and fast walk (bottom row), toe clearance during decline (left column), level walking (middle column) and incline (right column), and toe clearance during FES OFF (in green), FES FD (in red), FES OL (in orange) and FES CL conditions (in blue). The region where minimum toe clearance (Min TC) is expected, is highlighted in a grey background. Ankle angle variability is not shown for clarity. Vertical bars ($|$) indicate points in the gait cycle where mean differences between conditions are statistically significant (p$<$0.05).
• Figure 5: Mean knee angle during the swing phase for all subjects under the different conditions including toe clearance during slow walk (top row) and fast walk (bottom row), toe clearance during decline (left column), level walking (middle column) and incline (right column), and toe clearance during FES OFF (in green), FES FD (in red), FES OL (in orange) and FES CL conditions (in blue). The region where minimum toe clearance (Min TC) is expected, is highlighted in a grey background. Knee angle variability is not shown for clarity. Vertical bars ($|$) indicate points in the gait cycle where mean differences between conditions are statistically significant (p$<$0.05).