Method of analysis influences interpretations of sex-related differences in firing rates during prolonged submaximal isometric contractions

OBJECTIVE: This study examined motor unit (MU) firing rates during a prolonged isometric contraction of the vastus lateralis (VL) for females and males. METHODS: Surface electromyographic (sEMG) signals were recorded from the VL for eleven females and twelve males during a 45-second isometric trapezoid muscle actions at 40% of maximal voluntary contraction (MVC). For each MU, mean firing rate (MFR) was calculated for the initial and final 10-second epochs of the steady torque segment and regressed against recruitment threshold (RT, expressed as %MVC), as well as time at recruitment (T(REC), seconds). MFR was also averaged for each subject. RESULTS: Significant differences existed across epochs for the y-intercepts (P=0.009) of the MFR vs. T(REC) relationship, as well as the grouped MFR analysis (P<0.001); no differences were observed between epochs for the MFR vs. RT relationship. Significant differences existed between sexes for the grouped MFR analysis (P=0.049), but no differences were observed for the MFR vs. T(REC) or MFR vs. RT relationships. CONCLUSION: Analysis method may impact interpretation of firing rate behavior; increases in MU firing rates across a prolonged isometric contraction were observed in the MFR vs. T(REC) relationship and the grouped MFR analysis.