The effects of the menstrual cycle (MC) on exercise metabolism and performance are equivocal. PURPOSE: To investigate whether the MC impacted submaximal exercise oxygen consumption (VO2) and Critical Power (CP) in recreationally active, eumenorrheic females. METHODS: Eight eumenorrheic females (Age: 33±8 y, VO2max: 36.9±3.8 ml/kg/min) completed fasted exercise testing in 3 distinct phases of the MC (early follicular, late follicular, and mid luteal). MC phase was determined using calendar-based counting, ovulation test strips, and confirmed via serum hormones (estrogen and progesterone). For submaximal exercise, participants cycled for 20 min at 50% VO2max while VO2 and respiratory exchange ratio (RER) were measured with indirect calorimetry. Rating of perceived exertion (RPE) was measured with Borg 6-20 Scale. After submaximal exercise, participants rested for 10 min and performed a 3-min all-out test to assess CP. Power output (W) was continuously measured to determine CP, which was defined as mean W in final 30 sec of 3-min all-out test. Repeated measures ANOVA was used to assess differences in submaximal responses for VO2 and RER, and 3-min all-out CP. Friedman tests were used to assess differences in RPE. RESULTS: For submaximal cycling, VO2 did not differ across MC (early follicular: 1.27±0.22 L/min, late follicular: 1.27±0.20 L/min, mid-luteal: 1.27±0.22 L/min; p-value=0.98; effect size=0.01). However, RER was higher during late follicular (0.91±0.03) than early follicular (0.89±0.05) and mid-luteal (0.89±0.03) phases (p=0.12; effect size=0.30). RPE was lower during late follicular (11±2) compared to early follicular (13±2) and mid luteal (13±2)(p=0.04). For the 3-min all-out test, CP did not differ by MC phase (early follicular: 138±31 W, late follicular: 136±33 W, mid luteal: 136±31 W; p=0.52; effect size=0.07). CONCLUSIONS: These preliminary results in 8 recreationally active, eumenorrheic females suggest that submaximal exercise VO2 and CP do not differ across the MC, despite minor differences in submaximal substrate utilization and perceived exertion.



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