title: Influence of the contractile properties of muscle on motor unit firing rates during a moderate-intensity contraction in vivo.
abstract: It is suggested that firing rate characteristics of motor units (MUs) are influenced by the physical properties of the muscle. However, no study has correlated MU firing rates at recruitment, targeted force, or derecruitment with the contractile properties of the muscle in vivo. Twelve participants (age = 20.67 ± 2.35 yr) performed a 40% isometric maximal voluntary contraction of the leg extensors that included linearly increasing, steady force, and decreasing segments. Muscle biopsies were collected with myosin heavy chain (MHC) content quantified, and surface electromyography (EMG) was recorded from the vastus lateralis. The EMG signal was decomposed into the firing events of single MUs. Slopes and y-intercepts were calculated for 1) firing rates at recruitment vs. recruitment threshold, 2) mean firing rates at steady force vs. recruitment threshold, and 3) firing rates at derecruitment vs. derecruitment threshold relationships for each subject. Correlations among type I %MHC isoform content and the slopes and y-intercepts from the three relationships were examined. Type I %MHC isoform content was correlated with MU firing rates at recruitment (y-intercepts: r = -0.577; slopes: r = 0.741) and targeted force (slopes: r = 0.853) vs. recruitment threshold and MU firing rates at derecruitment (y-intercept: r = -0.597; slopes: r = 0.701) vs. derecruitment threshold relationships. However, the majority of the individual MU firing rates vs. recruitment and derecruitment relationships were not significant (P > 0.05) and, thus, revealed no systematic pattern. In contrast, MU firing rates during the steady force demonstrated a systematic pattern with higher firing rates for the lower- than higher-threshold MUs and were correlated with the physical properties of MUs in vivo.
authors.affiliation: Neuromechanics Laboratory, University of Kansas, Lawrence, Kansas;
authors.forename: Michael A
authors.initials: MA
authors.lastname: Trevino
authors.affiliation: Neuromechanics Laboratory, University of Kansas, Lawrence, Kansas; t.herda@ku.edu.
authors.forename: Trent J
authors.initials: TJ
authors.lastname: Herda
authors.affiliation: Applied Physiology Laboratory, University of Kansas, Lawrence, Kansas; and.
authors.forename: Andrew C
authors.initials: AC
authors.lastname: Fry
authors.affiliation: Applied Physiology Laboratory, University of Kansas, Lawrence, Kansas; and.
authors.forename: Philip M
authors.initials: PM
authors.lastname: Gallagher
authors.affiliation: Applied Physiology and Sports Medicine Laboratory, Kansas State University, Manhattan, Kansas.
authors.forename: John P
authors.initials: JP
authors.lastname: Vardiman
authors.affiliation: Neuromechanics Laboratory, University of Kansas, Lawrence, Kansas;
authors.forename: Eric M
authors.initials: EM
authors.lastname: Mosier
authors.affiliation: Neuromechanics Laboratory, University of Kansas, Lawrence, Kansas;
authors.forename: Jonathan D
authors.initials: JD
authors.lastname: Miller
mesh_terms: D000200:Action Potentials / Q000502:physiology*; D000293:Adolescent; D000843:Ankle Joint / Q000294:innervation; D004576:Electromyography; D006621:Hip Joint / Q000294:innervation; D006801:Humans; D007719:Knee Joint / Q000294:innervation; D008297:Male; D009046:Motor Neurons / Q000502:physiology*; D009119:Muscle Contraction / Q000502:physiology*; D018482:Muscle, Skeletal / Q000166:cytology* / Q000378:metabolism; D018995:Myosin Heavy Chains / Q000378:metabolism; D011999:Recruitment, Neurophysiological; D055815:Young Adult
chemical_list: D018995:Myosin Heavy Chains
keywords: motor unit firing rates; myosin heavy chain; onion skin control scheme; surface EMG decomposition; vastus lateralis