Background: Increases in skeletal muscle force generation are mediated by the recruitment of additional, progressively larger motor units (MU) and modulation of MU firing rate (FR) (Enoka, 2017). Males typically have larger muscles and greater muscle strength compared to females, and we have previously shown that females exhibit higher MU FR in contractions normalised to absolute strength. However, it remains unclear if these differences in MU FR are attributable to inherent sex characteristics or the substantial sex-based differences in absolute force output. The purpose of this study was to determine sex differences in individual MU features of the vastus lateralis in strength-matched males and females.
Methods: Twenty-four healthy participants (18-35 years), 12 males and 12 females, were recruited to this study, based on similar knee-extensor isometric strength (<4% mean difference). Vastus lateralis electromyography data were collected by intramuscular electrodes during 10% and 25% of maximum isometric voluntary contractions (MVC). Decomposition-based quantitative electromyography (DQEMG) software was used to identify individual motor unit potentials (MUPs) from each contraction, and their corresponding MUP trains. MU FR was assessed as the rate of MUP occurrences within a MUP train, and MUP area was taken as the total area within the MUP duration (onset to end). Multi-level mixed-effect linear regression models were used to investigate the effect of sex at each contraction level. Significance was assumed when p<0.05.
Results: There was no sex difference in maximum muscle torque (p=0.576). When compared to males, females had a 16% higher MU FR at 10% MVC (M:7.9 Hz; F: 9.2 Hz, p=0.002) and 8% higher at 25% MVC (M:8.7 Hz; F: 9.4 Hz, p=0.052). There was a significant interaction between sex and contraction level in MU FR (p=0.024) with males exhibiting a 10% increase (p<0.001) when moving from low to mid-level contraction, with only a 2% increase in females (p=0.488). Males exhibited a 27% (M: 880 mV·ms; F: 565 mV·ms, p=0.036) and 17% (M: 1114 mV·ms; F: 944 mV·ms, p=0.434) larger MUP area at 10% and 25% MVC, respectively, when compared to females. Both males and females showed significant increases in MUP area (24% and 50%, respectively) from low- to mid-level contractions (both p<0.001) with no interactions between sex and contraction level detected (p=0.5). 
Conclusions: Our findings demonstrate that females exhibit higher MU FR than males to generate absolute force, with a suppressed modulation of MU FR and greater increase in MUP size in females when moving from a low to mid-level contraction, highlighting divergent sex-based MU recruitment strategies to generate similar forces. Although assessed at the lower end of the contraction range of similar absolute forces, females rely more on additional MU recruitment and males rely more on MU FR modulation to increase force.
Ethics approval: This research was approved by the University of Nottingham Faculty of Medicine and Health Sciences Research Ethics Committee ((C16122016, 160-0121, 186-1812, 103-1809, 302-1903) and was conducted between 2019 and 2021 in accordance with Declaration of Helsinki.