Effect of low dose oral contraceptives on exercise performance.

OBJECTIVE to examine the effect of cycle phase or a low dose oral contraceptive on exercise performance in young women. METHODS As controls, 15 men were tested twice by a maximal treadmill test (Vo2 max) and by an endurance run 14 d apart to determine performance variability from causes other than hormonal fluctuations. Ten women ages 18-30 were then tested for Vo2 max and endurance in the same way in both the follicular and the luteal phase (random order, ovulation assessed by sonography). They were then randomly assigned to placebo (n = 3) or oral contraceptive (1 mg norethindrone and 35 micrograms ethinyl oestradiol) (n = 7) for 21 days. Tests were repeated during the first and third weeks of treatment. Vo2 max and endurance tests were compared in the men and control cycle of the women by using independent t tests on percent change. The data for both cycles in the women were analysed by repeated measures ANOVA. RESULTS There was no difference in per cent change in total test time, Vo2 max, or breathing frequency between the men and women in either test. Data obtained during the Vo2 max test revealed no difference between the follicular and luteal phases of the menstrual cycle for total test time [11.8 (SD 2.3) v 12.6 (2.3) min], Vo2 [41.6 (12.1) v 39.7 (11.4) ml.kg-1.min-1], or breathing frequency [26.8 (3.5) v 27.3 (9.9) breaths.min-1] respectively, or during the first and third weeks of treatment [total test time 12.0 (2.5) v 12.8 (2.4) min; Vo2 37.3 (7.4) v 41.0 (12.4) ml.kg-1.min-1; breathing frequency 27.8 (4.2) v 27.7 (3.4) breaths.min-1, respectively]. Data obtained during the endurance test revealed no difference between the follicular and luteal phase of the menstrual cycle for total test time [20.5 (15.7) v 16.2 (8.5) min], Vo2 [37.5 (9.4) v 32.9 (8.1) ml.kg-1.min-1], or breathing frequency [32.0 (6.0) v 33.2 (5.1) breaths.min-1, respectively], or during the first and third weeks of treatment [total test time 32.3 (34.9) v 30.6 (30.1) min; Vo2 33.9 (10.1) v 35.2 (8.6) ml.kg-1.min-1; breathing frequency 34.0 (5.9) v 34.8 (5.3) breaths.min-1, respectively]. CONCLUSIONS Neither cycle phase nor a low dose oral contraceptive containing 1 mg norethindrone adversely affects performance during a maximal treadmill test or endurance run.