Paula Radcliffe at the 2011 Berlin Marathon British runner Paula Radcliffe broke the marathon world record in 2003 and later told scientists that she accomplished the feat at the start of her period, which caused cramping and discomfort during her run. (Photo: 360b/Shutterstock)

It's been 45 years since Title IX of the Education Amendments of 1972 gave female athletes a better shot at equal inclusion in sports. Despite shrinking the gender gap in athletic programs though, women are still grossly underrepresented in exercise research — the research that athletes use to improve their performance.

Why do so many exercise studies exclude women? Some researchers suggest that it's because women have periods; the fluctuations in hormones during menstruation tend to mess up the data.

A review of sports and exercise research from 2011 to 2013, which included 1,382 studies and more than 6 million participants, found that women were represented in just 39 percent of studies. A 2016 editorial in the British Journal of Sports Medicine that attempted to explain that number concluded that "the complexities of the menstrual cycle are considered major barriers to the inclusion of women in clinical trials."

Why would a woman's period make such a difference? When researchers conduct a study, they attempt to control all of the variables except for the one that they're testing. In a study on, say, the effect of caffeine on athletic performance, researchers would have participants consume varying levels of caffeine while trying to keep the other variables as similar as possible. All the athletes would be in the same location and performing the same task and may even wear the same clothing in repeated tests. Researchers could then test parameters that would give an indication of fitness — such as heart rate and blood oxygen levels — and be relatively confident that any difference in their measurements was caused by the one variable they had changed and were measuring.

It's all about variables

Woman exercising How does a woman's menstrual cycle affect her sports performance? Researchers still don't know. (Photo: SerbBgd/Shutterstock)

But the fluctuation of hormones in a menstrual cycle can affect a woman's athletic performance, too. If researchers are attempting to control that variable, they would need to find a group of women at the same time in their cycles with similar hormonal levels, which would be hard to do. Another option that researchers have relied on is to use female participants who are on birth control pills and whose hormones are therefore more easily regulated.

But limiting data to one stage of the cycle, or using data only from women in whom hormone levels have been controlled, wouldn't paint an accurate picture about the overall athletic performance of women for one simple reason: Women compete during all stages of a menstrual cycle.

What's even more frustrating is that so little research has been done on how the menstrual cycle affects performance in the first place. In 2015, British tennis player Heather Watson made headlines when she attributed her defeat in the first round of the Australian Open to the fact that she had her period and was experiencing dizziness, nausea and fatigue. It was one of the first times that an athlete had opened up about her menstrual cycle and how it affected her performance.

For women who have had to fight to be considered equals in sport, it may seem taboo to bring up a subject that separates them from men. Taboo or not, female athletes need accurate information to make the same fine-tuning improvements that men can make with each new study. And that means embracing the menstrual cycle and finding ways to accurately measure its effect on athletic performance.

Hopefully, as more light is shed on the lack of female representation in exercise research, and as more athletes feel comfortable sharing information about how their periods affect their fitness, we'll get a better understanding of how women athletes can improve their performance. Then, the doors will really swing open.

Why are women so underrepresented in research on exercise?
Why aren't women counted in most exercise studies? Their periods tend to mess up the data.