Oxygen Uptake Kinetics During the Different Phases of the Menstrual and Oral Contraceptive Cycles

PURPOSE: To examine whether oxygen uptake (V̇O2) kinetics changes across the phases of the menstrual and oral contraceptive cycles.

METHODS: Fourteen highly active women who were either non-oral contraceptive users (n=7, 28±6 yrs.) or monophasic oral contraceptive users (n=7, 22±3 yrs.) participated in the study. The time-constant of the V̇O2 kinetics response (τV̇O2) was determined by ensemble-averaging the V̇O2 response measured during three consecutive step-transitions in work rate, from 20 Watts (W) to a moderate-intensity work rate of 80 W. Each step was six minutes in duration. The test was completed during the menstruation phase of the cycles (follicular phase for non-oral contraceptive users or “inactive pill” phase for oral contraceptive users) and repeated during the respective non-menstruating phase (luteal phase or “active pill” phase). An ovulation test was used to validate the menstrual cycle phase. A metabolic cart was used to continuously measure expired gas concentrations and ventilatory rates. A one-way repeated-measures ANOVA was used to compare the differences in V̇O2 kinetics across cycle phases between non-oral contraceptive and oral contraceptive users. Statistical significance was set at p<0.05.

RESULTS: The time constant for the adjustment of V̇O2 was affected by cycle phases, regardless of contraception use, whereby τV̇O2 was greater in the menstruation phases of the non-oral contraceptive and oral contraceptive cycles (24±7 s) compared to the non-menstruating phases (19±5 s) (p<0.05).

CONCLUSION: The speed of the V̇O2 kinetics response is affected by the phases of the menstrual and oral contraceptive cycles, such that a greater τV̇O2 is observed during the menstruation phase.