title: The relationship of motor unit size, firing rate and force.
abstract: OBJECTIVE
  Using a clinical electromyographic (EMG) protocol, motor units were sampled from the quadriceps femoris during isometric contractions at fixed force levels to examine how average motor unit size and firing rate relate to force generation.
  
  
  METHODS
  Mean firing rates (mFRs) and sizes (mean surface-detected motor unit action potential (mS-MUAP) area) of samples of active motor units were assessed at various force levels in 79 subjects.
  
  
  RESULTS
  MS-MUAP size increased linearly with increased force generation, while mFR remained relatively constant up to 30% of a maximal force and increased appreciably only at higher force levels. A relationship was found between muscle force and mS-MUAP area (r2 = 0.67), mFR (r2 = 0.38), and the product of mS-MUAP area and mFR (mS-MUAP x mFR) (r2 = 0.70).
  
  
  CONCLUSIONS
  The results support the hypothesis that motor units are recruited in an orderly manner during forceful contractions, and that in large muscles only at higher levels of contraction ( > 30% MVC) do mFRs increase appreciably. MS-MUAP and mFR can be assessed using clinical EMG techniques and they may provide a physiological basis for analyzing the role of motor units during muscle force generation.
authors.affiliation: Department of Neurology, Johns Hopkins Bayview Medical Center, Baltimore, MD 21224, USA.
authors.forename: R A
authors.initials: RA
authors.lastname: Conwit
authors.forename: D
authors.initials: D
authors.lastname: Stashuk
authors.forename: B
authors.initials: B
authors.lastname: Tracy
authors.forename: M
authors.initials: M
authors.lastname: McHugh
authors.forename: W F
authors.initials: WF
authors.lastname: Brown
authors.forename: E J
authors.initials: EJ
authors.lastname: Metter
mesh_terms: D000200:Action Potentials / Q000502:physiology; D000328:Adult; D000368:Aged; D000704:Analysis of Variance; D004576:Electromyography; D006801:Humans; D008875:Middle Aged; D009046:Motor Neurons / Q000502:physiology*; D009132:Muscles / Q000502:physiology*