Muscle Fiber Composition and Motor Unit Recruitment Patterns in Adult Males and Females

Abstract

It is unknown whether firing rate patterns differ between sexes in extended isometric contractions. Purpose: Therefore, the purposes of this study are to examine potential differences between untrained males and females for (1) motor unit (MU) activity during a single extended contraction (2) MU activity during a subsequent extended contraction, specifically the relationships between motor unit action potential amplitudes (MUAPAMPS) vs. recruitment threshold (RT), firing rates vs. RT, and firing rates versus MUAPAMPS, in addition to the root mean square of the electromyographic readings (EMGRMS). (3) To determine if % myosin heavy chain (MHC) isoform or muscle cross sectional area (mCSA) explain differences between two repetitions of extended isometric contractions. Methods: Ultrasonography was used to obtain subjects’ muscle cross sectional area (mCSA), subcutaneous fat (sFAT) and echo intensity (mEI) from 50% upper leg length of the vastus lateralis (VL). MU activity was analyzed from two extended isometric contractions at 40% maximum voluntary contraction (MVC) by the decomposition of the EMG signal from the surface of the skin. Muscle biopsies were also performed from the muscle belly of the VL, to be used for MHC analysis. Results: There were no significant differences between groups in age (female = 19.8 ± 1.3 yrs, male = 19.5 ± 1.2 yrs; P = 0.577) or body mass (female = 66.3 ± 14.14 kg, male = 79.35 ± 16.63 kg; P = 0.070), but there were significant differences in height (female = 165.21 ± 6.16 cm, male = 180.69 ± 8.31 cm; P < 0.001) and MVC strength (female = 125.9 ± 29.3 N, male = 209.1 ± 75.8 N; P = 0.003). No significant differences existed for mCSA between sexes (female = 19.02 ± 3.88 cm2, male = 26.52 ± 9.67 cm2; P = 0.069) There were no significant interactions for sex × % MHC isoform (P = 0.107), but there was a significant main effect for % MHC isoform (P = 0.028). Percent MHC I was greater (45.45 ± 9.53%) than % MHC IIA (37.16 ± 10.34%) collapsed across sexes. For EMGRMS, there was no significant interaction for sex × repetition (P = 0.987). There was a significant main effect for sex (P = 0.001), but there was no significant main effect for repetition (P = 0.447), with males having greater EMGRMS at steady force than the females collapsed across repetitions. For the MFR vs. RT, MUAPAMP vs RT, and MFR vs. MUAPAMP relationships, there were no significant two-way interactions (sex × repetition) for the slopes, y-intercepts, A terms, or B terms. However, for the slopes of the MFR vs. RT relationship, there were significant main effects for sex (P = 0.005) and repetition (P = 0.037), where the slopes were more negative for females (-0.276 ± 0.020 pps/%MVC) than males (-0.184 ± 0.024 pps/%MVC). The slopes were more negative for the first repetition (-0.264 ± 0.022 pps/%MVC) than the second repetition (-0.197 ± 0.22 pps/%MVC). There was also a main effect for sex for the y-intercepts (P = 0.020); females had greater y-intercepts (22.57 ± 0.44 pps) than the males (20.86 ± 0.543). For the MUAPAMP vs. RT relationship, there was a significant main effect for sex (P = 0.027) for the y-intercepts; the males had greater y-intercepts (0.023 ± 0.014 mV) than the females (0.015 ± 0.009 mV). For the MUAPAMP vs. MFR relationship, there was a significant main effect for sex (P = 0.040) for the B terms; males (-4.48 ± 0.52 pps/mV) had less negative B terms than females (-5.92 ± 0.43 pps/mV). Discussion: Although there were minimal sex-related differences observed for the MU relationships, this result was not entirely unexpected due to the lack of significant differences between sexes for %MHC isoform or mCSA. The more negative slopes and B terms exhibited by the females in the MFR vs. RT and MFR vs. MUAPAMP relationships indicate that females potentially must recruit a larger percentage of their MU pool to maintain the necessary force level. The lack of differences between groups for the changes in firing rates or MUAPAMPS from the 1st to 2nd repetition is most likely due to the similarities in % MHC isoform or mCSA between groups. However, there were changes in firing rates collapsed across sex from the 1st to 2nd repetition, which was likely a function of larger MUs participating earlier in the contraction.