Colin Miller1, Michael Trevino1, Trent Herda2, Adam Sterczala3, Jonathan Miller2, Mandy Parra4, Hannah Dimmick5

1Oklahoma State University, Stillwater, OK; 2University of Kansas, Lawrence, KS; 3University of Pittsburgh, Pittsburgh, PA; 4Baker University, Baldwin City, KS; 5University of Calgary, Calgary, AB

PURPOSE: The purpose of this study was to examine the effects of consecutive long duration contractions on motor unit (MU) derecruitment behavior of the vastus lateralis (VL) for sedentary individuals. METHODS: Thirteen females (20.69 ± 2.75 yrs) and nine males (20.00 ± 1.41 yrs) volunteered for this study. An electromyographic (EMG) sensor was placed over the VL. Each participant completed isometric maximal voluntary contractions (MVC) on an isokinetic dynamometer followed by two consecutive isometric trapezoidal submaximal contractions (40% MVC) of the right knee extensors. For the submaximal contractions, the torque was increased at a rate of 10% MVC/s to the deserved torque level for 45s followed by a decrease of 10% MVC/s to baseline. Ten seconds of rest was given between the submaximal contractions. Decomposition techniques were applied to the EMG signals to extract action potentials and the firing events of single MUs. For each MU, the recruitment (REC; %MVC) and derecruitment (DEREC; %MVC) thresholds were calculated and linear regressions were performed on the DEREC versus REC relationships for each individual to determine the slopes and y-intercepts (y-ints). Separate two-way mixed factorial ANOVAs (sex x repetition) examined the slopes and y-ints. Follow-up analyses included Bonferroni corrections and alpha was 0.05. RESULTS: For the slopes and y-ints, there were no significant (p > 0.05) two-way interactions or main effects for sex. However, there were main effects for repetition (REP). The slopes were greater (p = 0.030) for REP 1 (1.33 ± 0.50) than REP 2 (1.12 ± 0.43), whereas the y-ints were greater (p = 0.024) for REP 2 (1.27 ± 14.57) than REP 1 (-6.50 ± 20.39). CONCLUSION: Caution is warranted when interpreting the findings for the y-ints as half of the subjects exhibited negative values. Indeed, the slopes indicated that males and females derecruited MUs at higher torque values (slopes > 1) than the initial recruited torque levels for both REPS. However, the slopes significantly decreased during the second contraction, likely due to fatigue. Future research should investigate if endurance training can improve fatigue resistance and prevent a decrease in MU potentiation during a second, long duration contraction.

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