VisR Ultrasound for Monitoring Muscle Fatigue with FES-Induced and Volitional Isometric Contraction of the Rectus Femoris in Participants with Uninjured and Injured Spinal Cords.

PURPOSE: Hybrid exoskeletons for spinal cord injury rehabilitation combine functional electrical stimulation (FES) with robotic assistance, but muscle fatigue limits FES effectiveness. Improved fatigue measurements could improve exoskeleton control by optimizing assistance. This study evaluates Viscoelastic Response (VisR) ultrasound imaging as a method for monitoring muscle fatigue through changes in muscle stiffness.

METHODS

VisR ultrasound was used to assess the rectus femoris in five unimpaired participants and three with spinal cord injury (SCI). Changes in relative elasticity (RE) from baseline were measured during FES-induced and volitional contractions, as well as in post-contraction stiffness following FES stimulation. Both transverse and longitudinal transducer orientations were used for data collection to evaluate the impact of transducer orientation.

RESULTS

In control participants, percent changes in RE were strongly correlated with FES-induced torque, with no significant effect of transducer orientation. A similar relationship was observed in SCI participants when the transducer was oriented transversely but not longitudinally. In both control and SCI participants, percent changes in RE during volitional contraction were lower than those predicted by FES-based linear models. Post-contraction RE values did not differ significantly from pre-contraction values after FES.

CONCLUSION

VisR ultrasound effectively tracks muscle stiffness during isometric contractions, with percent changes in RE closely reflecting FES-induced torque output. Findings suggest VisR may provide relevant feedback on actively contracting muscles, supporting its potential as a tool for monitoring fatigue and muscle activation to inform adaptive control strategies in rehabilitative hybrid exoskeletons.