- Endurance athletes Note: severity of low BMD and menstrual dysfunction differ by kind of exercise and activity. For example, runners have a higher prevalence of menstrual irregularity than swimmers and cyclists (131). By limiting enrollment to endurance athletes, we will eliminate variability from the type of activity. Endurance training is defined as > 4 h of aerobic weight-bearing training of the legs or specific endurance training weekly, or > 20 miles of running weekly for a period of > 6 months in the last year.