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

2025-10-03

AI Summary

Conventional open-loop functional electrical stimulation (FES) for foot drop employs fixed-intensity stimulation, often causing premature muscle fatigue or insufficient assistance—thereby increasing fall risk. This paper proposes a closed-loop FES control strategy that dynamically modulates stimulation intensity based on real-time toe-off height feedback, enabling demand-driven, personalized gait assistance. The system integrates kinematic sensing, a custom closed-loop control algorithm, and FES hardware, and is validated across diverse walking conditions—including varying speeds and terrains. Results demonstrate robust maintenance of target toe-off height under all tested conditions, with a 32.7% average reduction in stimulation intensity. Crucially, the method preserves natural hip, knee, and ankle joint kinematics, enhancing both walking safety and user comfort. The core innovation lies in the first use of toe-off height as the primary closed-loop control variable, enabling physiologically responsive, precision stimulation modulation.

Technology Category

• Intelligent Robots: State Estimation
• Cognitive Modeling & Cognitive Systems: Adaptive Behavior
• Search and Optimization: Sampling/Simulation-based Search

Application Category

• Responsible Web: Machine-in-the-loop, human agency and autonomy
• User Modeling, Personalization and Recommendation: Federated recommendation systems and personalization
• Systems and Infrastructure for Web, Mobile and WoT: Energy management for devices in mobile Web and WoT environments

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.

Problem

• Developing closed-loop FES controller for foot drop management
• Addressing overstimulation and understimulation risks in current systems
• Ensuring adequate toe clearance with reduced muscle fatigue

Innovation

• Closed-loop FES controller adjusts stimulation dynamically
• Real-time toe clearance used for intensity adaptation
• Lower stimulation maintains toe clearance effectively