As a junior tennis player, American Shelby Rogers says the effect of the menstrual cycle on performance was rarely taken into consideration.
"I don't want to say it was taboo necessarily, but it was definitely not something talked about frequently, or openly," Rogers says.
"That was especially true if you had a male versus female coach."
But as a professional on the women's tennis tour, the 30-year-old — currently ranked 47th in the world — says talking about menstruation has become more "normalised".
"Tennis is really physically demanding, and if you're cramping or in a lot of pain, it's really tough to compete — some players have had to withdraw from matches because of it, and a lot are on birth control because they find it helpful with the symptoms," she says.
"If we can find ways to alleviate the symptoms, and understand how your body's going to respond to certain stressors during certain times, that would be huge.
"It's such a big factor when it comes to women's sport and how we perform."
The problem for elite athletes and everyday people alike, however, is that science knows very little about the menstrual cycle, especially when it comes to fitness and performance.
Women push for change
Exercise science has overwhelmingly neglected female biology. Of the total body of exercise science research, just 5 per cent of studies focus exclusively on women.
Dr Stacy Sims, an exercise physiologist and nutritional scientist in New Zealand, says that when it comes to research, women are often placed in the "too hard" basket.
"Scientific design is done through a male lens, and women have traditionally been considered delicate flowers," Sims says.
"There's a perception that we don't know enough about men [as it is] — so why would we want to study women? Then you have to take their menstrual cycle and hormones into account, which we don't know enough about."
Over the past five years, however, Sims says that's beginning to change.
She puts this down in part to elite athletes being increasingly open abut their periods and associated diseases like endometriosis, including tennis player Danielle Collins, who has spoken openly about the "agony" of her pelvic pain.
Sporting bodies are listening, too, with Wimbledon relaxing its all-white dress code to allow coloured undershorts, following player feedback that many were uncomfortable wearing white while having their periods.
Back home, the AFL followed suit, announcing it would ditch white shorts for AFLW players, to combat 'period stigma'.
Does tech hold the answer?
Broadly speaking, the menstrual cycle can be divided into two or more phases, including the follicular phase (prior to ovulation) and the luteal phase (post-ovulation).
Sometimes these stages are broken down again to include the menstrual phase (or "period", which happens at the beginning of the follicular phase, when the lining of the uterus is shed) and the ovulation phase.
Each phase is generally associated with changes in hormone levels, with estrogen rising prior to ovulation and dropping after.
Progesterone, meanwhile, tends to rise after ovulation, and is at its highest levels during the luteal phase.
Now there are also an array of apps which offer the option to "track" the menstrual cycle.
One such device is fitness "wearable" Whoop, which has partnerships with the Women's Tennis Association (WTA) and the Ladies' Professional Golf Association (LPGA).
It provides daily "recovery" scores to its users based on how ready their body is to take on "strain" (which includes exercise, but can also mean a stressful day of work).
To calculate a user's recovery score — green, yellow or red — the algorithm takes into account factors such as how well you have slept, your resting heart rate (RHR), and your heart rate variability (HRV), a well-established indicator of exercise "readiness".
The initial idea to track the impact of the menstrual cycle on recovery, Whoop's head of data science and research Emily Capodilupo says, came from surprising anecdotal feedback.
"Back in 2019, we started to get a lot of feedback that users always had green recovery scores — so that's the highest indication that their body is ready to perform — during the first part of their cycle, including their periods, which we thought was interesting," Capodilupo says.
Could menstruation be a time of 'peak' performance?
Whoop partnered with Dr Sims to see if they could start to pinpoint the influence hormonal changes could be having on menstruating users' recovery scores.
They analysed data on 4500 of their users (incorporating 13,535 menstrual cycles), with the findings broadly matching the anecdotal feedback they had received.
For those who were "naturally" cycling (i.e. not on the pill), recovery scores trended higher in the first half of their cycle (the follicular phase), before declining in the luteal phase.
"It was really evident from that paper that women are resilient to stress during the first part of their cycle, so even from day one [of their period] right up to ovulation," says Dr Sims.
In late 2021, Whoop announced a world-first menstrual "coaching'" function, seeking to apply these findings to its exercise recommendations.
"So after ovulation, our body becomes less resilient to stress," Dr Sims says. "We start to see a bit of a changeover, where there's evidence of less adequate recovery, or needing more recovery between hard efforts."
Capodilupo clarifies that this does not mean that menstruating users should not exercise during their luteal phase.
"But if you were going to align your training cycle with your menstrual cycle, you might want to have the hardest training in your follicular phase," she says.
Experts warn 'we don't have a consensus'
Not all experts, however, are convinced of Dr Sims' and Whoop's interpretation of the data.
This includes Professor Kirsty Elliott-Sale from Manchester Metropolitan University in the UK, who has researched female endocrinology, including the menstrual cycle, for more than 25 years.
"I don't think there's any software available now, any 'off-the-shelf' product, that we can use to track the cycle and verify its phases to give performance recommendations," Elliott-Sale says.
"We just don't have the research evidence to say, every woman in this phase is going to feel like this."
Associate Professor Clare Minahan from Griffith University, who is a member of the Australian Institute of Sport's Female Performance and Health Initiative's monitoring group, agrees.
"I would go so far as to say that we don't have a consensus on anything, in terms of how the menstrual cycle affects performance," Minahan says.
"We're so hungry for commercialisation and products, but they're not founded on consensus research."
Minahan clarifies that she is not saying Dr Sims and Whoop are necessarily "wrong". For example, she concedes that there may be some merit to the idea of phase-based training.
She points to a couple of "nicely controlled resistance exercise studies" which show that you may be able to make greater muscle gains if you train at a higher volume in your follicular phase, and pull back in your luteal phase.
"That's emerging as nice evidence to say, we should keep our eye on this, but it's not a consensus," she says.
Elliott-Sale says we have to be careful with jumping to conclusions.
"Would you change the face of medicine on a small handful of papers?" she asks.
"Personally, I think we need a meaningful number of high-quality independent studies that show the same findings.
"I think people are very well-intentioned, but for every 10 studies that show one effect when it comes to the menstrual cycle, there are the same number that completely contradict it."
Quality research lacking due to under-funding
Both experts agree that a major issue with our understanding of the menstrual cycle is not just a lack of research, but a lack of quality research thanks to chronic under-funding.
As Elliott-Sale puts it, good quality studies are "time-consuming" and costly, and require a sophisticated understanding of the nuances of female endocrinology.
Isolated findings are also notoriously difficult to generalise.
"There is so much variation, not just between women, but individual women can experience variability too," she says.
"So a woman might have a 27-day long cycle this time, then 30 days the next."
Then there's the large number of people whose hormonal profile differs from the "textbook" four-phase cycle described above.
For example, some women have what's called a "luteal phase deficiency" in which their ovaries do not produce as much progesterone in the second half of their cycle as would be expected.
Others may experience anovulation, which means they don't ovulate. This can happen in an ongoing way, or intermittently.
"There are probably dozens of different menstrual irregularities and dysfunction profiles, and those are just two of the most well-known," Elliott-Sale says.
A related issue is that menstrual "trackers" are unable to accurately verify where someone is in their cycle, and how that correlates to their levels of various hormones like estrogen and progesterone.
"Unless you're drawing blood and sending that off to a lab, you can't actually measure hormonal concentration," Elliott-Sale says.
Saliva and urine samples can also be used to estimate hormonal levels, but these are not always considered to be as accurate as blood tests, she says.
"So unless you're using something that is blood, or at least salivary-based, you can't predict with any accuracy where you are in your menstrual cycle.
"It's a very nuanced area, and I'm still learning every day."
'Individual' approach may be needed
Despite these issues, Elliott-Sale isn't prepared to do away with the idea of menstrual tracking entirely.
Instead, she recommends an "individual" approach.
"If you were working with someone like an athlete, I would tell them, once you've got a sense of the length of your cycle, and where those 'milestones' like ovulation are occurring, what you can do is track your symptoms," she says.
"Perhaps they always feel tired in a certain phase of their cycle, they could say, you know, I'm not feeling great, can we modify my training here? Or maybe they go to a nutritionist and look at changing their diet to optimise their energy levels."
"We definitely want people like athletes and coaches to consider menstrual cycles when it comes to performance," she says.
"If we don't, we just go back 20 to 40 years ago where it's just pushed under the carpet."
