title: Effects of fatigue on motor unit firing rate versus recruitment threshold relationships.
abstract: BACKGROUND
  The purpose of this study was to examine the influence of fatigue on the average firing rate versus recruitment threshold relationships for the vastus lateralis (VL) and vastus medialis.
  
  
  METHODS
  Nineteen subjects performed ten maximum voluntary contractions of the dominant leg extensors. Before and after this fatiguing protocol, the subjects performed a trapezoid isometric muscle action of the leg extensors, and bipolar surface electromyographic signals were detected from both muscles. These signals were then decomposed into individual motor unit action potential trains. For each subject and muscle, the relationship between average firing rate and recruitment threshold was examined using linear regression analyses.
  
  
  RESULTS
  For the VL, the linear slope coefficients and y-intercepts for these relationships increased and decreased, respectively, after fatigue. For both muscles, many of the motor units decreased their firing rates.
  
  
  CONCLUSIONS
  With fatigue, recruitment of higher threshold motor units resulted in an increase in slope for the VL.
authors.affiliation: Department of Health and Exercise Science, University of Oklahoma, 1401 Asp Avenue, Norman, Oklahoma 73019-6081, USA. mattstock@ou.edu
authors.forename: Matt S
authors.initials: MS
authors.lastname: Stock
authors.forename: Travis W
authors.initials: TW
authors.lastname: Beck
authors.forename: Jason M
authors.initials: JM
authors.lastname: Defreitas
mesh_terms: D000200:Action Potentials / Q000502:physiology*; D000293:Adolescent; D000328:Adult; D000465:Algorithms; D000704:Analysis of Variance; D004576:Electromyography; D005260:Female; D006801:Humans; D007537:Isometric Contraction / Q000502:physiology; D008297:Male; D009046:Motor Neurons / Q000502:physiology*; D018763:Muscle Fatigue / Q000502:physiology*; D018482:Muscle, Skeletal / Q000503:physiopathology*; D011999:Recruitment, Neurophysiological / Q000502:physiology*; D012044:Regression Analysis; D012815:Signal Processing, Computer-Assisted; D055815:Young Adult