The purpose of this study was to examine whether swimming performance was affected by acute hormonal fluctuation within a monophasic
oral contraceptive (OC) cycle. Six competitive swimmers and water polo players completed a 200-m time trial at 3 time points of a single OC cycle: during the consumption phase (CONS), early (WITH1), and late in the withdrawal phase (WITH2). Split times and
stroke rate were recorded during the time trial, and heart rate, blood
lactate,
glucose, and pH were measured after each performance test. Resting endogenous serum
estradiol and
progesterone concentrations were also assessed. No significant differences were observed between phases for body composition, 200-m swim time, mean
stroke rate, peak heart rate, or
blood glucose (p > 0.05). The mean peak blood
lactate was significantly lower during WITH2 (9.9 ± 3.0 mmol·L(-1)) compared with that of CONS (12.5 ± 3.0 mmol·L(-1)) and mean pH higher during WITH2 (7.183 ± 0.111) compared with that of CONS (7.144 ± 0.092). Serum
estradiol levels were significantly greater during WITH2 compared with that during WITH1 and CONS, but there was no difference in serum
progesterone levels. These results demonstrate that for monophasic OC users, cycle phase does not impact the 200-m swimming performance. There was a reduction in blood
lactate and an increase in pH during the withdrawal phase, possibly because of an increase in fluid retention, plasma volume, and cellular
alkalosis. Therefore, female 200-m swimmers taking a monophasic OC need not be concerned by the phase of their cycle with regard to competition and optimizing performance. However, coaches and scientists should exercise caution when interpreting blood
lactate results obtained from swimming tests and consider controlling for cycle phase for athletes taking an OC.