Evoked EMG versus muscle torque during fatiguing functional electrical stimulation-evoked muscle contractions and short-term recovery in individuals with spinal cord injury.

This study investigated whether the relationship between muscle torque and m-waves remained constant after short recovery periods, between repeated intervals of isometric muscle contractions induced by functional electrical stimulation (FES). Eight subjects with spinal cord injury (SCI) were recruited for the study. All subjects had their quadriceps muscles group stimulated during three sessions of isometric contractions separated by 5 min of recovery. The evoked-electromyographic (eEMG) signals, as well as the produced torque, were synchronously acquired during the contractions and during short FES bursts applied during the recovery intervals. All analysed m-wave variables changed progressively throughout the three contractions, even though the same muscle torque was generated. The peak to peak amplitude (PtpA), and the m-wave area (Area) were significantly increased, while the time between the stimulus artefact and the positive peak (PosT) were substantially reduced when the muscles became fatigued. In addition, all m-wave variables recovered faster and to a greater extent than did torque after the recovery intervals. We concluded that rapid recovery intervals between FES-evoked exercise sessions can radically interfere in the use of m-waves as a proxy for torque estimation in individuals with SCI. This needs to be further investigated, in addition to seeking a better understanding of the mechanisms of muscle fatigue and recovery.

The effectiveness of FES-evoked EMG potentials to assess muscle force and fatigue in individuals with spinal cord injury.

The evoked electromyographic signal (eEMG) potential is the standard index used to monitor both electrical changes within the motor unit during muscular activity and the electrical patterns during evoked contraction. However, technical and physiological limitations often preclude the acquisition and analysis of the signal especially during functional electrical stimulation (FES)-evoked contractions. Hence, an accurate quantification of the relationship between the eEMG potential and FES-evoked muscle response remains elusive and continues to attract the attention of researchers due to its potential application in the fields of biomechanics, muscle physiology, and rehabilitation science. We conducted a systematic review to examine the effectiveness of eEMG potentials to assess muscle force and fatigue, particularly as a biofeedback descriptor of FES-evoked contractions in individuals with spinal cord injury. At the outset, 2867 citations were identified and, finally, fifty-nine trials met the inclusion criteria. Four hypotheses were proposed and evaluated to inform this review. The results showed that eEMG is effective at quantifying muscle force and fatigue during isometric contraction, but may not be effective during dynamic contractions including cycling and stepping. Positive correlation of up to r = 0.90 (p < 0.05) between the decline in the peak-to-peak amplitude of the eEMG and the decline in the force output during fatiguing isometric contractions has been reported. In the available prediction models, the performance index of the eEMG signal to estimate the generated muscle force ranged from 3.8% to 34% for 18 s to 70 s ahead of the actual muscle force generation. The strength and inherent limitations of the eEMG signal to assess muscle force and fatigue were evident from our findings with implications in clinical management of spinal cord injury (SCI) population.

Evoked EMG and muscle fatigue during isokinetic FES-cycling in individuals with SCI.

PURPOSE: This study investigated whether muscle fatigue during functional electrical stimulation (FES)-induced cycling was associated with changes occurring in evoked electromyographic signals (eEMG, M-waves) in individuals with spinal cord injury. We also explored the effects of recovery intervals between exercise sessions on the relationship between eEMG and muscle torque. METHODS: Eight individuals with spinal cord injury performed three FES-cycling sessions of 15-min duration, with 5 min of recovery between them. The quadriceps muscles were electrically stimulated as the prime agonist to produce cycling. Pedal torques and surface eEMG signals were synchronously processed and recorded for offline analysis. RESULTS: Large Torque decreases (20-44%) were observed in the first 5 min of cycling during the three exercise bouts, while changes of similar magnitude did not occur on any of the M-wave time-series (less than 19%). Between 5 and 15 min of cycling, muscle fatigue lowered the plateau baselines of Torque (ranging from 41% to 62%), M-wave peak-to-peak amplitude (PtpA) and Area (ranging from 60% to 98%) time-series, yet the magnitudes of these reductions were not consistent between them. CONCLUSION: We concluded that muscle fatigue during FES-cycling was not associated with, nor could be predicted by, eEMG signals. Nonetheless, the consistency between M-waves and Torque time-curves in their direction of change clearly warrants further investigation.