A Hybrid Knee Exoskeleton Using Real-Time Ultrasound-Based Muscle Fatigue Assessment

Ultrasound-based state assessment of the human muscle during rehabilitation and its integration into a hybrid exoskeleton comprising an functional electrical stimulation (FES) system and a powered orthosis are emerging research areas. This article presents results from the first experimental demonstration of a hybrid knee exoskeleton that uses ultrasound-derived muscle state feedback to coordinate electrical motors and FES. A significant contribution of the article is to integrate a real-time ultrasound image acquisition and processing framework into a recently derived switching-based feedback control of the hybrid knee exoskeleton. As a result, the contractility response of the quadriceps muscle to the FES input can be monitored in vivo in real-time and estimate FES-induced muscle fatigue changes in the muscle. The switched controller’s decision-making process can then use the estimated muscle fatigue to compensate or replace the FES-stimulated muscle power with an electrical motor, thus avoiding extensive stimulation of the fatigued muscle. The experimental results suggest a potential application in the rehabilitation of neurological disorders like spinal cord injuries and stroke.