Effect of endurance exercise on ovulation in the rat

This study was designed to evaluate individual variation in ovarian responses to exercise. Rats ran on a treadmill (15% incline) at a rate of 26 m.min-1 for 90 min every day for 14-21 d; control rats were placed on a stationary treadmill for the same period of time. Based on daily vaginal cytology, 9 of 17 exercising rats had abnormal ( > 5 d) estrous cycles in the first cycle of training compared to only 2 of 19 controls. Throughout the training period, the length of estrous cycles of 8 of the 17 exercising rats were relatively unaffected; the mean length of completed cycles was 5.1% +/- 0.4 (SE) d. However, of those length of completed cycles was 5.1 +/- 0>4 (SE) d. However, of those thought to have ovulated based on vaginal cytology, fewer exercising than control rats had new corpora lutea on their ovaries. In exercising rats acyclic periods of up to 21 d were associated with serum progesterone levels similar to those of the diestrus phase of the estrous cycle (mean, 29 +/- 4 SE nmol.l-1). We conclude that treadmill running extends the period of progesterone secretion, delaying ovulation in a susceptible population of rats, and that vaginal cornification may not be an accurate indication of ovulation in these animals.