Sex differences in the modulation of the motor unit discharge rate leads to reduced force steadiness.

The purpose of this study was to evaluate the relationship between the variability in the motor unit inter-pulse interval and force steadiness at submaximal and maximal force outputs between the sexes. Twenty-four male and twenty-four female participants were recruited to perform isometric dorsiflexion contractions at 20, 40, 60, 80, and 100% maximum voluntary contraction (MVC). Tibialis anterior myoelectric signal was recorded by an intramuscular electrode. Females had lower force steadiness (coefficient of variation of force (CoV-Force), 27.3%, p<0.01) and a greater coefficient of variation of motor unit action potential inter-pulse interval (CoV-IPI), compared to males (9.6%, p<0.01). There was no significant correlation between the normalized CoV-IPI and CoV-Force (r=0.19, p>0.01), but there was a significant repeated measures correlation between the raw scores for root-mean-square force error and the standard deviation of motor unit discharge rate (r=0.65, p<0.01). Females also had a greater incidence of doublet discharges on average across force levels (p<0.01). The sex differences may result from motor unit behaviors (ie, doublet and rapid discharges, synchronization, rate coding or recruitment), leading to lower force steadiness and greater CoV-IPI in females. Novelty Bullets: • Sex differences in force steadiness may be due to neural strategies • Females have lower force steadiness compared to males • Greater incidence of doublet discharges in females may result in lesser force steadiness.