Exercise and menstruation have a complex relationship, with various elements affecting just how often (or not) you have a period. The ideal is to achieve the middle ground: not too much and not too little exercise. A bonus is that exercise, in moderation, is believed to help relieve menstrual cramps.
The researcher examined the effect of exercise on a group of young, healthy women with normal menstrual cycles. In the first week after menstrual bleeding began, the women were given two exercises to do, and repeated them in the week after they ovulated. These times were chosen because the levels of the hormones oestrogen and progesterone increase toward the end of the menstrual cycle.