MENSTRUAL CYCLE PHASE EFFECTS ON EXERCISE THERMOREGULATION AND PERFORMANCE IN THE HEAT

Hormone fluctuations across the menstrual cycle (MC) may influence physiological responses. This study examined whether thermoregulatory measures differ across MC phases, and if these changes impact running performance in the heat. Ten eumenorrheic adults completed two treadmill runs in simulated conditions (wet bulb globe temperature 31.7 ± 1.4°C) during early follicular (EF) and midluteal (ML) phases. First 45min was a constant self-selected light pace followed by a 15min performance test for distance. Core temperature (Tc), mean skin temperature (Ts), heart rate (HR), perceived exertion (RPE), thermal perception (THERM), and comfort perception (COMF) were assessed every 5min. Sweat perception (SP), sweat loss (SL), fluid consumption, session RPE/THERM/COMF, and distance covered (DIST) were measured at completion. Serum samples were assayed for estrogen and progesterone to confirm MC phases. Phase × time ANOVA assessed physiological and perceptual measures. Paired t-tests assessed phase differences in session RPE/THERM/COMF, SP, SL, and DIST. Results indicated that Tc, Ts, HR, RPE, THERM, and COMF showed main effect of time (p < 0.001 for all), while only HR showed main effect of phase (p = 0.022, ηp2 = 0.50). COMF showed a phase-time interaction (p = 0.046, ηp2 = 0.19). Tc was higher during ML at baseline and across the bout despite no main effect of phase (p > 0.05). No differences were observed for SP, SL, fluid intake, session perceptions, or DIST (EF: 1.69 ± 0.28mi, ML: 1.69 ± 0.35mi) (p > 0.05 for all). Overall, Tc, HR and COMF are elevated in ML, with perceived discomfort becoming more pronounced across time, but do not influence exercise performance in the heat. Athletes may continue to exercise per usual without concern of phase effects.