Effects of menstrual cycle phase and oral contraceptive use on intermittent exercise.

Five women using low-dose, monophasic oral contraceptive (OC) agents (OC group) and ten normally menstruating women (Non-OC group) performed a treadmill protocol to determine the effect of OCs and the menstrual cycle (MC) on intermittent exercise performance and some commonly used metabolic markers. The Non-OC group were tested once in the mid-follicular phase (MFP) and once in the late luteal phase (LLP) of the MC, while the OC group performed their first test within 1 week of taking the OC (T1) and their second test 1 week later (T2). Despite performance time being the same in both groups [mean (SD), Non-OC group: 77.7 (14.9) s versus OC group: 77.7 (21.1)s], plasma ammonia concentration ([NH3]pl) was higher in the Non-OC group when compared to the OC group throughout recovery (P < 0.05). No differences were found in blood lactate (BLa), maximum heart rate or aural temperature (Tau) between groups. Within the Non-OC group Tau increased with exercise in both phases (P < 0.05), however Tau was higher in the LLP at rest [36.1 (0.3) degrees C) and 1 min post-exercise [37.1 (0.6) degrees C), when compared to the MFP [35.8 (0.3) and 36.9 (0.7) degrees C, rest and 1 min post-exercise respectively, P < 0.05]. Within the OC group T1 resulted in a higher peak BLa [11.2 (0.4) mmol/l] and [NH3]pl (143.0 (26.2) Umol/l] when compared to T2 [BLa, 9.6 (0.9); [NH3], 119.4 (48.1), P<0.05]. These results suggest that: (1) exercise performance does not vary between the MFP and the LLP of the MC, nor does it appear to be affected by the number of days using the OC, and (2) an altered metabolism occurs both between groups (Non-OC versus OC) and within the OC group.