Towards real-time muscle fatigue and recovery markers using a wearable continuous wave Doppler ultrasound system

Functional electrical stimulation (FES) is commonly used in physical rehabilitation, bypassing the central nervous system to activate motor neurons directly. However, the unnatural muscle recruitment pattern induced by FES causes rapid muscle fatigue, greatly reducing the muscle's ability to generate force. Currently there exists no reliable, real time indicator for FES-induced muscle fatigue. We believe that signs of muscle fatigue can be inferred from medical ultrasound. Previously we investigated tissue Doppler imaging (TDI) to study muscle physiology associated with muscle potentiation and fatigue. Here, we expand on that research using continuous wave (CW) Doppler ultrasound to create a wearable, low power muscle fatigue monitor. We are investigating this system to work with a hybrid FES exoskeleton designed to use the patient's own muscles with FES with the added stability of an exoskeleton. CW ultrasound indicated that the duration of muscle recruitment decreased from 129.0 ms to 51.7 ms for the same FES as the muscle fatigued. Further, we showed that muscle twitch duration and velocity correlate with twitch force, a marker of fatigue recovery, using TDI and CW. These fatigue and recovery measures can be used to inform the exoskeleton controller to coordinate FES and electric motors for producing gait.