Search

This study examined the effects of continuous endurance training on motor unit (MU) mean firing rates (MFR), percent myosin heavy chain (%MHC) isoforms, and muscle cross-sectional area (mCSA) of the vastus lateralis (VL). Twelve females completed 5-weeks of continuous cycling-training (CYC), while 8 females were controls (CON). Participants performed maximal voluntary contractions (MVCs) and 40% MVCs of the knee extensors before (PRE) and after the 5-week treatment period at the same absolute pre-treatment submaximal torque (POST

Despite ample evidence that females are weaker and possess smaller muscle cross-sectional 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 years) and 12 females (age 21 ± 1 years) 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 0.05) during the 10% MVC. For the 70% MVC, the y-intercepts from the MUAPAMPS vs. RT relationships were greater (p 0.05) in the slopes. Therefore, smaller CSAs and weaker MVCs are likely the result of smaller higher-threshold MUs for females.

It is common practice to examine motor unit (MU) activity according to mean firing rate (MFR) and action potential amplitude (MUAPAMP) vs. recruitment threshold (RT) relationships during isometric trapezoidal contractions. However, it is unknown whether the rate of torque development during the linearly increasing torque phase affects the activity of MUs during such contractions. Sixteen healthy males and females performed two isometric trapezoidal muscle actions at 40% of maximum voluntary contraction (MVC), one at a rate of torque development of 5% MVC/s (SLOW40) and one at 20% MVC/s (FAST40) during the linearly increasing torque phase. Surface electromyography (EMG) was recorded from the vastus lateralis (VL) via a 5-pin surface array sensor and decomposed into action potential trains of individual MUs, yielding MFRs and MUAPAMP which were regressed against RT separately for each contraction. Surface EMG amplitude recorded from leg extensors and flexors was used to quantify muscle activation and coactivation. MFR vs. RT relationship slopes were more negative (P = 0.003) for the SLOW40 (− 0.491 ± 0.101 pps/%MVC) than FAST40 (− 0.322 ± 0.109 pps/%MVC) and the slopes of the MUAPAMP vs. RT relationships (P = 0.022, SLOW40 = 0.0057 ± 0.0021 mV/%MVC, FAST40 = 0.0041 ± 0.0023 mV/%MVC) and muscle activation of the extensors (P