The Effects of Menstrual Cycle Phase on Performance in Endurance Runners

Introduction: The menstrual cycle is a widely overlooked and understudied component of female athlete training. The cycle is divided into the follicular and mid-luteal phases and is guided by the rise and fall of estrogen and progesterone. Menstrual cycle phase may have a performance effect on eumenorrheic female endurance runners, though conflicting evidence exists. The purpose of this study was to examine the effect of menstrual cycle phase on performance and associated physiological parameters in endurance runners. Methods: In this randomized cross-sectional study, five well-trained eumenorrheic female endurance runners completed eight weeks of data collection. Testing occurred during the early follicular (EF) and mid-luteal (ML) phases. Each subject completed two VO₂max tests, two 5 km time trials (TT) with blood lactate tests pre- and post-, and three intravenous blood collections analyzing estrogen, progesterone, and cortisol. Urine progesterone and ovulation tests were used to confirm phase. ML estrogen and progesterone, VO₂max, heart rate (HR), respiratory exchange ratio (RER), and minute ventilation (VE) during each VO₂max stage, ventilatory threshold 1 (VT1) and ventilatory threshold 2 (VT2), 5 km TT, blood lactate pre- and post-5 km TT, and plasma cortisol concentrations were analyzed between the EF and ML phases. Correlations were run between estrogen, progesterone, blood lactate, and cortisol with associated variables. Results: No significant differences were found in VO₂max, HR, RER, or VE during VO₂max stages, VT1 and VT2, 5 km TT, or blood lactate pre- and post-5 km TT between EF and ML phases. There was a significant difference in EF and ML plasma cortisol concentrations (EF: 11.1 ± 1.3 μg/dL, ML: 8.8 ± 2.4 μg/dL; p = 0.04), as well as a significant correlation between ML plasma cortisol and progesterone concentrations (p = 0.005). No significant correlations were found between any other measures. Conclusions: The findings of this study did not support the hypothesis. No significant differences were found between exercise performance or associated parameters between EF and ML phases, though a significant difference was found between EF and ML plasma cortisol concentrations, with EF cortisol measuring significantly higher. The findings of the current study raise questions as to how menstrual cycle-based cortisol fluctuations may affect performance in endurance athletes and should provoke future investigation.