Sex-Related Differences in Motor Unit Behavior of the Vastus Lateralis During a High Intensity Contraction


Alex Olmos, Michael Trevino, Trent Herda, Adam Sterczala, Jonathan Miller, Mandy Parra, Hannah Dimmick, Jake Broeckel

PURPOSE: The purpose of this study was to examine motor unit (MU) firing rates and muscle activation of the vastus lateralis (VL) between sedentary males and females during a 70% maximal voluntary isometric contraction (MVIC). METHODS: Eleven males (mean ± SD; Age: 20.36 ± 1.50 yrs, Height: 179.73 ± 4.13 cm, Body Mass: 78.84 ± 13.29 kg) and ten females (Age: 21.10 ± 6.08 yrs, Height: 166.40 ± 5.70 cm, Body Mass: 64.88 ± 15.03 kg) participated in this study. A surface electromyographic (sEMG) 5-pin sensor was placed over the VL. Participants performed three MVICs of the knee extensors on an isokinetic dynamometer and the greatest value was recorded. Participants then performed an isometric trapezoid muscle action at 70% of the participants greatest MVIC. For the isometric trapezoid muscle action, the torque was increased at a rate of 10% MVIC/s to the desired force level, which was maintained for 12 s followed by a decrease of 10% MVC/s to baseline. Decomposition techniques were applied to the sEMG signals to extract individual action potentials and firing events for single MUs. For each MU, the recruitment threshold (RT) and mean firing rates (MFR) were calculated. Only MUs with firing accuracies > 90% were used for analysis. Linear regressions were performed on the MFR vs. RT relationships to determine y-intercepts (pulses per second [pps]) and slopes (%MVC/pps) for each participant. EMG amplitude (RMS) during the plateau of the trapezoid was normalized to MVC (N-EMGRMS). Three independent-sample t-test were used to examine sex-related differences NEMGRMS and the slopes and y-intercepts from the relationships. RESULTS: For the y-intercepts there were no significant differences (P = 0.184; males = 32.84 ± 5.01 pps, females = 32.60 ± 6.65 pps). For the slopes, there was a significant difference (P = 0.003) between the males (-0.487 ± 0.126 %MVC/pps) and the females (-0.335 ± 0.101 %MVC/pps). For N-EMGRMS, there was a significant difference (P = 0.003) between males (71.74 ± 7.06 %) and females (93.22 ± 15.63 %). CONCLUSION: The results indicate sex-related differences in motor unit firing rate behavior and muscle activation for the VL during high intensity contractions. Research should further investigate if these differences can be elucidated by MU size and the fiber area of the VL.

ACKNOWLEDGEMENTS: This study was supported by the Doctoral Research Grant fund from the National Strength and Conditioning Association, Colorado Springs, CO.

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