Pelvic floor myoelectric excitation and force output across menstrual-cycle phases

Abstract Objective To determine whether menstrual-cycle (MC) phase is associated with changes in pelvic floor muscle (PFM) function, focusing on myoelectric excitation (intravaginal sEMG) at rest and during standardized contraction tasks, and on mechanical strength assessed by dynamometry and digital palpation. Methods Prospective observational, within-subject study in 23 healthy nulliparous eumenorrheic women (18–35 years) not using hormonal contraceptives. Each participant was assessed in early follicular (EFP), ovulatory (OP), and mid-luteal (MLP) phases, confirmed using urinary luteinizing hormone (LH) testing, basal temperature, and transvaginal ultrasonography. Outcomes included Modified Oxford Scale (MOS), Pelvibex® dynamometry (passive, active, net force), and intravaginal sEMG (MVC plus Glazer protocol: baseline, phasic, tonic, endurance, baseline post-endurance). Friedman tests with Holm-adjusted post hoc comparisons and Spearman correlations were applied. Results Voluntary contraction sEMG (MVC, phasic, tonic, endurance) showed no significant phase differences (p > 0.05). Resting sEMG differed by phase: baseline average and peak amplitude was lower in OP vs. EFP (pHolm = 0.012; pHolm = 0.018, respectively). Strength outcomes (MOS and dynamometric measures) did not differ across phases (all p > 0.05). sEMG exhibited strong within-phase coherence and cross-phase stability, while baseline myoelectric excitation correlated inversely with force measures. Conclusions MC phase was associated with subtle modulation of baseline PFM myoelectric excitation (most consistently lower at ovulation), without detectable phase-related changes during voluntary contractions or in mechanical strength. Baseline sEMG amplitude showed inverse associations with active force, suggesting that higher residual excitation may be associated with reduced mechanical strength. Overall, electrophysiological and force-based outcomes should be interpreted cautiously.