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

•The soleus muscle H-reflex is larger after fatigue induced by supra-maximal repetitive stimulation.•“Asynchronous” EMG bursts, consistent with fluctuations in muscle force, were not prevalent during supra-maximal stimulation.•Supra-maximal stimulation, administered at physiologic frequencies (e.g. 15 Hz), is a useful strategy to administer fixed levels of physiologic stress (force) to paralyzed tissues.

ObjectiveElectrical 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.MethodsWe 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).ResultsBefore 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 ∼ 1 s after the stimulus train.ConclusionsFatigue 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.SignificanceThese 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.