Sex-related differences in motor unit behavior are influenced by myosin heavy chain during high- but not moderate-intensity contractions

Aims

Motor unit recruitment and firing rate patterns of the vastus lateralis (VL) have not been compared between sexes during moderate- and high-intensity contraction intensities. Additionally, the influence of fiber composition on potential sex-related differences remains unquantified.

Methods

Eleven males and 11 females performed 40% and 70% maximal voluntary contractions (MVCs). Surface electromyographic (EMG) signals recorded from the VL were decomposed. Recruitment thresholds (RTs), MU action potential amplitudes (MUAP_AMP), initial firing rates (IFRs), mean firing rates (MFRs), and normalized EMG amplitude (N-EMG_RMS) at steady torque were analyzed. Y-intercepts and slopes were calculated for MUAP_AMP, IFR, and MFR versus RT relationships. Type I myosin heavy chain isoform (MHC) was determined with muscle biopsies.

Results

There were no sex-related differences in MU characteristics at 40% MVC. At 70% MVC, males exhibited greater slopes (p = 0.002) for the MUAP_AMP, whereas females displayed greater slopes (p = 0.001–0.007) for the IFR and MFR versus RT relationships. N-EMG_RMS at 70% MVC was greater for females (p < 0.001). Type I %MHC was greater for females (p = 0.006), and was correlated (p = 0.018–0.031) with the slopes for the MUAP_AMP, IFR, and MFR versus RT relationships at 70% MVC (r = −0.599–0.585).

Conclusion

Both sexes exhibited an inverse relationship between MU firing rates and recruitment thresholds. However, the sex-related differences in MU recruitment and firing rate patterns and N-EMG_RMS at 70% MVC were likely due to greater type I% MHC and smaller twitch forces of the higher threshold MUs for the females. Evidence is provided that muscle fiber composition may explain divergent MU behavior between sexes.