Sex-related differences in mCSA and MUAP of the higher-threshold MUs were likely the result of larger muscle fibres expressing type II characteristics for males.
Previous investigations report no changes in motor unit (MU) firing rates during submaximal contractions following resistance training. These investigations did not account for MU recruitment or examine firing rates as a function of recruitment threshold (REC). Therefore, MU recruitment and firing rates in chronically resistance-trained (RT) and physically active controls (CON) were examined. Surface electromyography signals were collected from the first dorsal interosseous during isometric muscle actions at 40% and 70% maximal voluntary contraction (MVC). For each MU, force at REC, mean firing rate (MFR) during the steady force, and MU action potential amplitude (MUAP) were analyzed. For each individual and contraction, the MFRs were linearly regressed against REC, whereas, exponential models were applied to the MFR versus MUAP and MUAP versus REC relationships with the y-intercepts and slopes (linear) and A and B terms (exponential) calculated. For the 40% MVC, the RT had less negative slopes (p = 0.001) and lower y-intercepts (p = 0.006) of the MFR versus REC relationships and lower B terms (p = 0.011) of the MUAP versus REC relationships. There were no differences in either relationship between groups for the 70% MVC. During the 40% MVC, the RT had a smaller range of MFRs and MUAP in comparison with the CON, likely because of reduced MU recruitment. The RT had lower MFRs and recruitment during the 40% MVC, which may indicate a leftward shift in the force-frequency relationship, and thus require less excitation to the motoneuron pool to match the same relative force.
Despite ample evidence that females are weaker and possess smaller muscle crosssectional areas (CSAs) compared to males, it remains unclear if there are sex-related differences in the properties of motor units (MU). Eleven males (age=22±3 yrs) and twelve females (age=21±1yrs) performed isometric trapezoid muscle actions at 10% and 70% of maximal voluntary contraction (MVC). Surface electromyography signals were recorded and decomposed into MU action potential (AP) waveforms and firing instances. Average MUAP amplitudes (MUAPAMPS), mean firing rates (MFRs), initial firing rates (IFRs) and recruitment thresholds (RT) were calculated for the 10% MVC while MUAPAMPS, IFRs, and MFRs were regressed against RT for the 70% MVC. Ultrasonography was used to measure CSA of the first dorsal interosseous (FDI). Males had greater CSAs (p<0.001; males=2.34±0.28 cm 2 , females=1.82±0.18 cm 2 ) and MVC strength (p<0.001; males=25.9±5.5 N, females=16.44±2.5 N).No differences existed for MUAPAMPS, IFRs, MFRs, or RTs (p>0.05) during the 10% MVC. For the 70% MVC, the y-intercepts from the MUAPAMPS vs. RT relationships were greater (p<0.05) for the males (males=-0.19±0.53mV; females=-0.78±0.75 mV) while the inverse was true for the MFR vs. RT relationships (males=31.55±6.92 pps, females=38.65±6.71 pps) with no differences (p>0.05) in the slopes. Therefore, smaller CSAs and weaker MVCs are likely the result of smaller higher-threshold MUs for the females.
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