A question of rate? Girl-women differences in motor unit firing rate

To investigate age-related differences in motor unit (MU) activation patterns in females during submaximal knee extensions.

Fourteen young women (22.1 ± 1.5 yrs) and 10 girls (9.8 ± 0.8 yrs) participated in the study. Following the determination of maximal strength (MVC), participants completed trapezoidal isometric contractions to 70%MVC and sEMG was recorded from the vastus lateralis. sEMG signals were decomposed, and MU firing rate (MUFR) and recruitment threshold (RT) were calculated. Group differences in MU activation patterns were assessed using multilevel modelling.

Girls' MVC was significantly lower than young women's, even after accounting for body size (2.0 ± 0.4 and 2.6 ± 0.8 Nm/body mass, respectively). The y-intercept of the MUFR-RT relationship was significantly lower in girls compared with the young women (estimate = 4.38pps; p < 0.05). There were no group differences in the MUFR-RT slope, reflecting lower MUFR in girls among low and high-threshold MUs. Lastly, the highest RT at which MUs were identified was significantly (p = 0.04) lower in girls (53.4 ± 7.2%MVC) compared with young women (60.5 ± 8.2%MVC). Overall, girls had lower MUFR and recruited their MUs over a smaller range of RTs compared with women.

Together, these results suggest that girls and young women utilize different MU activation schemes when performing high-intensity knee extensions. Specifically, compared with women, girls may have to recruit more MUs over a narrower recruitment range due to their lower MUFRs.