Are Responses to Submaximal Exercise Less Reproducible in Females vs Males Due to the Menstrual Cycle?

Exercise-science research often excludes young eumenorrheic females due to the presence of a menstrual cycle (MC) and lack of knowledge on how much variability the MC adds to the physiological responses to exercise. PURPOSE: To determine if the metabolic response to submaximal exercise is less reproducible in females with a menstrual cycle compared to males. We hypothesized females would exhibit greater variability in the metabolic response to submaximal-intensity exercise across the MC than males across a similar timespan. METHODS: Eumenorrheic, endurance-trained females (n=10, age=24.1 ± 5.59) performed two 5-minute bouts of constant-load, moderate-intensity cycling at three time points across the MC (early follicular, late follicular and mid-luteal phases). Endurance-trained males (n=10, age= 29.5 ± 9.18) performed the same tests ~10 days apart to mimic the time between the phases of the MC. Steady-state V̇O2, heart rate (HR), ventilation (V̇E), respiratory exchange ratio (RER), carbohydrate oxidation (CHO %), tidal volume (Vt), and rating of perceived exertion (RPE) were measured during the fifth minute of each bout on two consecutive days for each phase. Average values for each phase were analyzed using a mixed-model ANOVA. RESULTS: No phase-by-sex interactions (P2, HR, RPE, V̇E, and Vt. The coefficient of variation for these variables was generally low (2-10%) and did not differ between sexes for any of these variables (P>0.268). RER and CHO % exhibited a tendency for an effect of phase on CHO oxidation (P=0.056), which seems to be driven by greater CHO oxidation during the early follicular phase of females (P=0.093). Nevertheless, the coefficient of variation was still low (CV=~2%) and did not differ between sexes (P=0.697). CONCLUSION: The metabolic response to moderate-intensity exercise is largely unaffected by the MC of eumenorrheic, endurance-trained females. These females should not be excluded from research on the assumption that the MC will add excessive variability. Variables that might require further analysis include RER and CHO %.