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Article Title

Myosin Heavy Chain Expression Influences Motor Unit Behavior During High- But Not Low-Intensity Isometric Contractions

Abstract

Stephanie Sontag1, Alex Olmos1, Trent Herda2, Philip Gallagher2, Andrew Fry2, Jake Deckert3, Adam Sterczala4, Jonathan Miller2, Mandy Parra5, Hannah Dimmick6, Michael Trevino1

1Oklahoma State University, Stillwater, OK, 2The University of Kansas, Lawrence, KS, 3Gonzaga University, Spokane, WA, 4The University of Pittsburg, Pittsburg, PA, 5Baker University, Baldwin City, KS, 6The University of Calgary, Calgary, AB

PURPOSE: To examine the influence of percent (%) type I myosin heavy chain (MHC) isoform expression on the slope and y-intercept values from the mean firing rate (MFR) versus motor unit (MU) recruitment threshold (RT) relationships of the vastus lateralis (VL) for sedentary individuals during a 40% and 70% maximal voluntary contraction (MVC). METHODS: Seven men (mean ± SD; age: 21.0 ± 1.5 yrs, height: 179.71 ± 5.25 cm, body mass: 81.77 ± 15.59 kg) and seven women (age: 24.0 ± 6.9 yrs, height: 165.14 ± 7.10 cm, body mass: 58.70 ± 15.37 kg) volunteered for this study. An electromyographic (EMG) sensor was placed on the VL. Each participant performed three isometric MVCs of the leg extensors on an isokinetic dynamometer and the highest value was used. Participants then performed separate isometric trapezoidal muscle actions with torque increasing at a rate of 10% MVC/s to the desired torque level, which was maintained for 12 s followed by a decrease of 10% MVC/s to baseline. Action potentials and firing events of single MUs were extracted from EMG signals using decomposition techniques. The MFRs and RTs were calculated for each MU. MUs with accuracies > 90% were used for analysis. Linear regressions were performed to determine the slopes and y-intercepts of the MFR versus RT relationships. Subjects gave a muscle biopsy of the VL after testing. SDS-PAGE was used to determine the % type I MHC isoform expression. Pearson’s product moment correlations were used to determine relationships among % type I MHC isoform expression with the slope and yintercept values from the MFR versus RT relationships. Alpha was set at 0.05. RESULTS: Pearson’s product moment correlations were significant when comparing % type I MHC isoform expression to the slope from the MFR versus RT relationship at 70% MVC (p = 0.022, r = 0.604) but not at 40% MVC (p = 0.066, r = 0.504). There were no significant correlations when comparing % type I MHC isoform expression to the y-intercept from the MFR versus RT relationship at 70% MVC (p = 0.138, r = -0.416) or at 40% MVC (p = -0.179, r = 0.318). CONCLUSION: The slope values from the MFR versus RT relationships were correlated with % type I MHC isoform expression at high- but not low-intensities. Higher contraction intensities may be necessary for the physical properties of the MU pool to influence MU behavior in untrained individuals.

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