title: Synchronization of low- and high-threshold motor units.
abstract: BACKGROUND
  We examined the degree of synchronization for both low- and high-threshold motor unit (MU) pairs at high force levels.
  
  
  METHODS
  MU spike trains were recorded from the quadriceps during high-force isometric leg extensions. Short-term synchronization (between -6 and 6 ms) was calculated for every unique MU pair for each contraction.
  
  
  RESULTS
  At high force levels, earlier recruited motor unit pairs (low-threshold) demonstrated relatively low levels of short-term synchronization (approximately 7.3% extra firings than would have been expected by chance). However, the magnitude of synchronization increased significantly and linearly with mean recruitment threshold (reaching 22.1% extra firings for motor unit pairs recruited above 70% MVC).
  
  
  CONCLUSIONS
  Three potential mechanisms that could explain the observed differences in synchronization across motor unit types are proposed and discussed.
authors.affiliation: University of Oklahoma, 1401 Asp Avenue, Norman, Oklahoma, 73019.
authors.forename: Jason M
authors.initials: JM
authors.lastname: Defreitas
authors.forename: Travis W
authors.initials: TW
authors.lastname: Beck
authors.forename: Xin
authors.initials: X
authors.lastname: Ye
authors.forename: Matt S
authors.initials: MS
authors.lastname: Stock
mesh_terms: D000200:Action Potentials / Q000502:physiology*; D000293:Adolescent; D004576:Electromyography / Q000379:methods; D005260:Female; D006801:Humans; D007537:Isometric Contraction / Q000502:physiology*; D008297:Male; D052097:Quadriceps Muscle / Q000502:physiology*; D011999:Recruitment, Neurophysiological / Q000502:physiology*; D055815:Young Adult
keywords: Synchrony; common inputs; decomposition; motor neuron; recruitment threshold