Exercise intensity. An important factor in the etiology of menstrual dysfunction?

The objective of this case control study was to compare training activity in female long distance runners with irregular (IR, n =13) and regular (R, n =16) menstrual function, especially with regard to exercise intensity. Serum estradiol concentration during the follicular phase was considered normal in the R subjects (31±6 pg/1). In contrast, the IR runners were clearly hypoestrogenic (11±1.6 pg/1). The two groups had similar V o_2max' anaerobic threshold (AT) and maximal heart frequency (HF_max). In both groups AT was found at 88% of HF_max' The two groups had similar mean race results in half marathons during the previous year. The number of running contests was, however, twice as high in the IR group. A light portable pulse recorder was used to monitor heart rate during 1 week of regular training activity, and during a controlled high intensity work-out. During a week with normal training activity, an equal amount of training at high intensity levels (>85% HF_max) was performed by the two groups. The amount of training at lower intensities (<85% HF_max) was, however, more extensive in the group with irregular menstrual function. During the controlled self-administered, high intensity work-out, the amounts of training at and above AT were equal in the two groups. In conclusion: no difference in extent of high intensity training between long distance runners with regular and irregular menstrual function could be found in the present study group. The IR runners did, however, have significantly more training activity at lower intensity levels, and more frequent race participation.