title: The change in motor unit firing rates at de-recruitment relative to recruitment is correlated with type I myosin heavy chain isoform content of the vastus lateralis in vivo.
abstract: OBJECTIVE
  To investigate the change in motor unit (MU) firing rates (FR) at de-recruitment relative to recruitment and the relation to % type I myosin heavy chain isoform content (type I %MHC) of the vastus lateralis (VL) in vivo.
  
  
  METHODS
  Ten subjects performed a 22-s submaximal isometric trapezoid muscle action that included a linearly increasing, steady force at 50% maximal voluntary contraction, and linearly decreasing segments. Surface electromyographic signals were collected from the VL and were decomposed into constituent MU action potentials trains. A tissue sample from the VL was taken to calculate type I %MHC. The y-intercepts and slopes were calculated for the changes (Δ) in FR at de-recruitment (FRDEREC ) relative to FR at recruitment (FRREC ) vs. FRREC relationship for each subject. Correlations were performed between the y-intercepts and slopes with type I %MHC.
  
  
  RESULTS
  The majority of MUs had greater FRDEREC than FRREC . The y-intercepts (r = -0.600, P = 0.067) were not significantly correlated, but the slopes (r = -0.793, P = 0.006) were significantly correlated with type I %MHC.
  
  
  CONCLUSIONS
  The majority of the motoneuron pool had greater FRDEREC than FRREC , however, individuals with higher type I %MHC had a greater propensity to have MUs with FRREC > FRDEREC as indicated by the slope values. Overall, the contractile properties of the muscle (MHC) could partially explain the differences in MU firing rates at de-recruitment relative to recruitment. Thus, suggesting the fatigability of the muscle influences the alterations in MU firing rates from recruitment to de-recruitment.
authors.affiliation: Neuromechanics Laboratory, Department of Health, Sport and Exercise Sciences, University of Kansas, Lawrence, KS, USA.
authors.forename: T J
authors.initials: TJ
authors.lastname: Herda
authors.affiliation: Neuromechanics Laboratory, Department of Health, Sport and Exercise Sciences, University of Kansas, Lawrence, KS, USA.
authors.forename: J D
authors.initials: JD
authors.lastname: Miller
authors.affiliation: Neuromechanics Laboratory, Department of Health, Sport and Exercise Sciences, University of Kansas, Lawrence, KS, USA.
authors.forename: M A
authors.initials: MA
authors.lastname: Trevino
authors.affiliation: Neuromechanics Laboratory, Department of Health, Sport and Exercise Sciences, University of Kansas, Lawrence, KS, USA.
authors.forename: E M
authors.initials: EM
authors.lastname: Mosier
authors.affiliation: Applied Physiology Laboratory, Department of Health Sport and Exercise Sciences, University of Kansas, Lawrence, KS, USA.
authors.forename: P M
authors.initials: PM
authors.lastname: Gallagher
authors.affiliation: Neuromechanics Laboratory, Department of Health, Sport and Exercise Sciences, University of Kansas, Lawrence, KS, USA.
authors.forename: A C
authors.initials: AC
authors.lastname: Fry
authors.affiliation: Applied Physiology Laboratory, Department of Health Sport and Exercise Sciences, University of Kansas, Lawrence, KS, USA.
authors.forename: J P
authors.initials: JP
authors.lastname: Vardiman
mesh_terms: D000200:Action Potentials / Q000502:physiology; D024722:Cardiac Myosins / Q000378:metabolism*; D004576:Electromyography; D005260:Female; D006801:Humans; D007537:Isometric Contraction; D008297:Male; D009046:Motor Neurons / Q000378:metabolism; D018482:Muscle, Skeletal / Q000378:metabolism*; D018995:Myosin Heavy Chains / Q000378:metabolism*; D020033:Protein Isoforms; D011999:Recruitment, Neurophysiological / Q000502:physiology*; D055815:Young Adult
chemical_list: C501641:MYH7 protein, human; D020033:Protein Isoforms; D024722:Cardiac Myosins; D018995:Myosin Heavy Chains
keywords: firing rate; motor unit; myosin heavy chain; vastus lateralis