Oestrogen and progesterone are the primary female sex hormones that have fluctuating concentrations throughout a menstrual cycle. As they each have neural excitatory and inhibitory capabilities [1] they represent an attractive human model to study widely reported performance differences across the cycle. For example, in the quadriceps, motor unit firing rate (MUFR) was highest during the early follicular phase when progesterone was at its lowest concentration [2]. A potential mechanism to explain these differences is the magnitude of motoneuronal persistent inward currents (PICs) which enable the amplification and prolongation of MU firing to facilitate muscle [3]. Furthermore, PIC amplitude is regulated via descending monoaminergic inputs, primarily serotonin and norepinephrine, both of which may be responsive to hormonal fluctuations.

The purpose of this multi-site study across four universities is to estimate PIC amplitudes across three phases of an eumenorrheic menstrual cycle (early follicular, pre-ovulation and mid-luteal phase). At each experimental visit, venous blood samples were drawn to quantify plasma levels of estradiol and progesterone. High-density surface electromyography (HDsEMG) recorded motor unit activity from the tibialis anterior during ramped isometric contractions peaking at 30% of maximal voluntary contraction. HDsEMG signals were decomposed into individual MU spike trains using blind source separation algorithms. PIC magnitudes were estimated via motor unit discharge hysteresis (ΔF) and the motor unit discharge nonlinearity with respect to torque (brace height). Statistical significance was accepted at p<0.05.

Preliminary data demonstrates ΔF was higher in the mid-luteal phase (5.84 ± 0.35 pps), compared to early follicular (5.45 ± 0.34 pps) and pre-ovulation (5.37 ± 0.35 pps; χ2 = 8.14, p = 0.017), respectively. Brace height was lower in the pre-ovulation phase (0.38 ± 0.016; χ2 = 22.95, p < 0.001) compared to early follicular (0.422 ± 0.015) and mid-luteal phase (0.45 ± 0.016).

This results herein suggest that the magnitude of PICs alters across the differing phases of the menstrual cycle which may be related to the altering effects of fluctuating hormones on monoaminergic drive to motoneurons. These changes in neuromuscular mechanisms may have important implications for physical performance in differing phases of the menstrual cycle, and may partly explain the variability in performance measures across the cycle. However, the data provided herein are preliminary and further research is necessary to determine any definitive conclusions.