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Article Title

THE INFLUENCE OF TYPE I MYOSIN HEAVY CHAIN EXPRESSION ON MOTOR UNIT FIRING RATES OF THE VASTUS LATERALIS IN VIVO

Abstract

Michael A. Trevino1, Eric M. Mosier1, Jonathan D. Miller1, Andrew C. Fry1, Philip M. Gallagher1, John P. Vardiman2, and Trent J. Herda1. 1University of Kansas, Lawrence, Kansas, 2Kansas State University, Manhattan, Kansas; e-mail: mtrevino@ku.edu

PURPOSE: Differences in motor unit (MU) behavior as a result of chronic training have previously been reported. It is hypothesized that MU behavior is regulated by the physical properties of a muscle rather than the central nervous system. Thus, it has been suggested that differences in MU firing rates between training statuses were likely due to differences in the physical properties of the MU, such as, percent myosin heavy chain [%MHC] expression. No study has correlated MU control strategies during a voluntary contraction with MHC expression in vivo. METHODS: Twelve individuals (age = 20.91 ± 2.30 yrs, weight = 70.76 ± 14.47 kg) volunteered for this investigation. Participants performed 3 isometric maximal voluntary contractions of the leg extensors on an isokinetic dynamometer followed by an isometric trapezoid muscle action at 40% MVC. An electromyographic (EMG) sensor was placed over the vastus lateralis (VL). EMG signals were decomposed to extract action potentials and firing events of single MUs. Only MUs with > 90% accuracies were used for further analysis. Recruitment (REC) thresholds and mean firing rates (MFR) were calculated for each MU. MFR was calculated as the average value of the MFR trajectory during steady force. Subjects gave a muscle biopsy of the VL. Type I %MHC expression was determined by SDS-PAGE. Linear regressions were performed to determine the slopes and y-intercepts of the MFR versus REC relationships. Pearson product-moment correlations were used to determine the relationship between type I %MHC expression with the slopes and y-intercepts. Alpha was set at 0.05. RESULTS: Pearson’s product moment correlations were significant between the type I %MHC expression and the slopes (P = 0.001, r = 0.844) from the MFR versus REC relationships, but not the y-intercepts (P = 0.826, r = -0.071). CONCLUSION: Individuals with a greater percentage of type I %MHC expression had greater firing rates of the higher-threshold MUs at the targeted force level than individuals with a lower percentage of type I %MHC expression. It is plausible that the firing rates of the higher-threshold MUs are lower in individuals with greater percentages of type II MHC isoform content as a result of greater twitch forces. This study supported the hypothesis that the MU control scheme is regulated by the physical properties of the muscle.

Funding provided by the General Research Fund from the University of Kansas, Lawrence, KS.

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