The purpose of the present investigation is to examine the changes in isometric strength and motor unit (MU) firing rates of the vastus lateralis (VL) and vastus medialis (VM) in response to 6 wk of high-load (HL; n = 10, 4 females) and low-load blood flow restriction resistance exercise training (RET) (LL-BFR; n = 10, 5 females). MU firing rates and recruitment threshold relationships were analyzed from surface electromyographic (sEMG) recordings of the leg extensors during submaximal isometric contractions performed at 10%, 40%, and 70% maximal voluntary contraction (MVC) pre- and post-training. Linear mixed-effects models were used to assess the influence of recruitment threshold, contraction intensity, muscle, sex, and training group on MU firing rates. Isometric strength increased in response to training (P = 0.010, d = 0.49), with no significant interaction for training groups. Firing rates remained unchanged in response to interventions. For the VL, females displayed greater firing rates regardless of recruitment position during the 10% MVC, whereas later-recruited MUs during the 40% and 70% MVC were higher, with no difference in earlier-recruited MU. For the VM, females exhibited greater firing rates of the higher-threshold MUs across all submaximal intensities. Interestingly, males displayed greater firing rates of the earlier-recruited MUs of the VM across all submaximal intensities. Thus, there were nonuniform sex-related differences in MU firing rates for the VL and VM. In conclusion, similar increases occurred for isometric strength following 6 wk of LL-BFR or HL RET in absence of an increase in firing rates of the leg extensors.NEW & NOTEWORTHY Motor unit (MU) firing rates did not differ in response to high-load (HL) or low-load blood flow restriction (LL-BFR) training when accounting for individual variability, contraction intensities, and recruitment position. Contributing to the divergent literature on sex-related differences in firing rates, highlighting nonuniform differences across muscle and contraction intensity. In addition, firing rates differ between VL and VM across parameters. To our knowledge, no prior investigation has compared firing rates of both leg extensor muscles in response to resistance exercise training (RET).
Bass et al. (Wed,) studied this question.