Fatigue modulates synchronous but not asynchronous soleus activation during stimulation of paralyzed muscle

Objective: Electrical stimulation over a motor nerve yields muscle force via a combination of direct and reflex-mediated activation. We determined the influence of fatigue on reflex-mediated responses induced during supra-maximal electrical stimulation in humans with complete paralysis. Methods: We analyzed soleus electromyographic (EMG) activity during repetitive stimulation (15 Hz, 125 contractions) in 22 individuals with complete paralysis. The bout of stimulation caused significant soleus muscle fatigue (53.1% torque decline). Results: Before fatigue, EMG at all latencies after the M-wave was less than 1% of the maximal M-wave amplitude (% MaxM). After fatigue there was a fourfold (p < 0.05) increase in EMG at the H-reflex latency; however, the overall magnitude remained low (<2% change in % MaxM). There was no increase in "asynchronous" EMG similar to 1 s after the stimulus train. Conclusions: Fatigue enhanced the activation to the paralyzed soleus muscle, but primarily at the H-reflex latency. The overall influence of this reflex modulation was small. Soleus EMG was not elevated during fatigue at latencies consistent with asynchronous activation. Significance: These findings support synchronous reflex responses increase while random asynchronous reflex activation does not change during repetitive supra-maximal stimulation, offering a clinical strategy to consistently dose stress to paralyzed tissues. Published by Elsevier Ltd. on behalf of International Federation of Clinical Neurophysiology.