title: Fatigue modulates synchronous but not asynchronous soleus activation during stimulation of paralyzed muscle.
abstract: 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 ∼ 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.
  
  
  CONCLUSIONS
  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.
authors.affiliation: Department of Physical Therapy and Rehabilitation Science, Carver College of Medicine, The University of Iowa, Iowa City, IA, USA. richard-shields@uiowa.edu
authors.forename: Richard K
authors.initials: RK
authors.lastname: Shields
authors.forename: Shauna
authors.initials: S
authors.lastname: Dudley-Javoroski
mesh_terms: D000328:Adult; D000368:Aged; D004558:Electric Stimulation; D004599:Electric Stimulation Therapy; D004576:Electromyography; D005260:Female; D006181:H-Reflex / Q000502:physiology*; D006801:Humans; D008297:Male; D008875:Middle Aged; D018763:Muscle Fatigue / Q000502:physiology*; D018482:Muscle, Skeletal / Q000503:physiopathology*; D010243:Paralysis / Q000175:diagnosis / Q000209:etiology / Q000503:physiopathology*; D011930:Reaction Time; D012018:Reflex / Q000502:physiology; D013119:Spinal Cord Injuries / Q000150:complications / Q000503:physiopathology*; D019415:Torque; D055815:Young Adult
keywords: Asynchronous EMG; H-reflex; Soleus; Spinal cord injury