In theory, that’s the idea behind syncing the four phases of our menstrual cycles with our fitness regimens. While the fluctuations in estrogen and progesterone influence fertility, they can also impact other bodily functions, including energy levels, the way we process food, and more. Fitness menstrual cycle syncing is about taking the phase of our cycle—and the accompanying hormone levels—into consideration when choosing a workout.

“Women often feel very different during the distinct phases of the cycle, so syncing fitness may help some women with body awareness, symptom relief, personal goals, and overall mood,” says Shannon DeVore, MD, an assistant professor in the department of obstetrics and gynecology at the NYU Langone Fertility Center, and a member of the P.volve clinical advisory board. P.volve launched a menstrual-cycle-linked fitness regimen called Phase & Function in 2021.

Just as in many fitness innovations, cycle syncing came from trying to optimize the performance of professional athletes—the U.S. Women’s National Soccer team famously announced in 2019 that they had been using menstrual cycle syncing to prepare for the World Cup. But research to undergird these pro-athlete strategies has been rare, since a menstrual cycle typically disqualifies people who ovulate from sports research, leading to the vast majority of sports science study participants being men.

However, research is picking up around the world, and places like the Stanford Female Athlete Science And Translational Research (FASTR) Program are specifically working to close that research gender gap. Insights are finding their way into the mainstream, too. Today, the hashtag #cyclesyncingworkout has 11.3 million views on TikTok, and “cycle syncing workouts” was a 2022 Google top trending search term, meaning that it had significantly more search interest in 2022 than in previous years. Along with P.volve, other top fitness programs, including [solidcore], Kayla Itsines’s Sweat, Nike, Tonal, and more have put out regimens, guides, and advice on the topic. After noticing the trend percolating, MindBody added the question of whether respondents structure their workouts based on their menstrual cycle to its 17,000-person wellness trends report survey, and found that 35 percent of women aged 18 to 50 said they do, and that number was even higher for millennials and members of Gen Z, at 38 and 39 percent, respectively.

The trend may be a result of a confluence of factors. The first is technological—the proliferation of period tracking apps have made it easier and much more common to be aware of the phases of your menstrual cycle. The next is scientific, with calls for a much-needed push to close that research gap between men and women in physiological research—and that the aforementioned technology is making this research easier. Finally, more people are exploring whether or not hormonal birth control is right for them, which means more people may be tuning into their “natural” rhythms.

But when we call something “natural,” we run the risk of elevating it as an “essentialist” female quality, and giving it an outsized influence on the role hormones and periods play in someone’s whole personhood.

“I think it’s all about empowerment,” Dr. Devore says. “But a little bit of it feels like, if we’re reduced to these hormones, are we in control of any of our behaviors?” It’s the kind of thinking that could put “us back into the ’50s,” she points out.

“A little bit of it feels like, if we’re reduced to these hormones, are we in control of any of our behaviors?” —Shannon DeVore, MD

Researchers think the need to understand how hormones impact our bodies, including potential benefits to our athletic performance, is what’s actually essential.

“It is amazing that we have these great hormones in our body that we can use to our benefit,” says Jacky Forsyth, PhD, an associate professor of exercise physiology at Staffordshire University and a medical expert for Flo, a period tracking app. She has been studying and advising professional athletes on cycle syncing and exercise for the last 20 years. “It’s a biological function,” she says. “Why not look at it?”

What are menstrual-cycle-syncing workouts?

When Dr. Forsyth works with coaches of women’s athletics teams looking to explore the idea of menstrual cycle syncing, she often has to begin at the same place: premenstrual syndrome (PMS), and the stereotypical symptoms that come with it.

“The (often) male coaches might say, ‘Oh yes, well I’m aware of certain symptoms associated with the menstrual cycle,’” Dr. Forsyth recalls of a lot of her first conversations.

But the idea encompasses a lot more than PMS, because our hormones are active day-in and day-out, not just around our periods—and not just around our ovaries and uteruses.

“There are estrogen receptors right throughout the body that develop in a variety of different tissues,” says Christine Yu, journalist and author of the forthcoming book Up to Speed: The Groundbreaking Science of Women Athletes. “As hormones rise and fall, they actually have a lot of effects throughout the body, so their effects are not just solely isolated to our reproductive system.”

Hormones don’t fluctuate in the same way for everyone, and that’s especially true if you are on hormonal birth control, like the pill. Taking synthetic estrogen and progestin (a form of progesterone) prevents your body from ovulating, which eliminates the hormonal surges that might govern a menstrual cycle-informed workout plan.

However, if you aren’t on hormonal birth control, those surges might be something you want to pay attention to. So menstrual-cycle-syncing workouts involve choosing and modulating your activity based on where you are in your cycle, and the accompanying hormone levels and ratios of each phase.

“Some of the research that is out there seems to suggest that the body responds differently to different levels and ratios of hormones,” Yu says. “The idea of cycle syncing is really this sense of, Can we actually take advantage of those fluctuations in hormones so that we can perform better, eke out some additional intensity, gain some more muscle, gain whatever measures of fitness, recover better if we do certain things that better align with the hormonal phase in which you are in?”

How can hormones impact fitness?

There are some easily understandable applications of menstrual cycle fitness syncing. Fatigue is a commonly reported premenstrual (or late luteal phase) symptom, because you have a deficit of estrogen and higher levels of progesterone. Or you might have a lot of energy during your follicular phase, when estrogen is surging, because estrogen is actually a steroid. So one version of menstrual cycle fitness syncing involves tuning into your mood and energy levels, and busting out higher effort or lower energy workouts accordingly.

But it can get a lot more granular than that.

Dr. Forsyth works with teams throughout Europe who are beginning to implement different versions of the method. There is some evidence that our hormones can impact not just our energy and mood, but our ability to perform.

Specifically, some studies show that people are able to build more muscle in the follicular phase (since estrogen is a steroid, remember?), so this phase would be an ideal time for strength training. Hormones also impact the way that we access our energy (food) stores. While we typically rely on carbohydrates for energy, during the luteal phase, our body turns more readily to fat. That’s why this might be a good time for steady state cardio as opposed to exercise that relies on bursts of energy.

“There’s so much interesting research coming out and people looking at this,” Yu says. “But we’re kind of getting ahead of ourselves.”

A meta-analysis that evaluated 51 studies was not able to conclude that the menstrual cycle has a demonstrable impact on performance. Yu explains that while some studies found that, for example, yes, the follicular phase with its estrogen surge can be an optimal time to build muscle, other studies found no such effects.

A key word to understanding challenges in this research is the word “can,” because how hormones impact the factors that could influence our exercise varies widely from person to person.

“[Some people] will probably be more successful in their plan if they sync it up and follow how their energy levels are feeling,” Dr. Devore says. “But then other women have no symptoms at all and no changes.”

That variability is part of what makes subjecting the theory of cycle syncing to the rigors of the scientific method so difficult. Other challenges include finding large enough sample sizes of subjects with regular periods who are not on birth control, as well as standardizing the data that’s collected across studies (though there is a current push for this standardization). However, Dr. Forsyth, who has contributed much to this body of research (beginning with her PhD in the early 2000s), believes that what meta-analysts describe as a lack of high-quality studies doesn’t mean that the impacts aren’t observable, and ultimately provable, with enough research.

“It’s sometimes hard to actually get a piece of research which is considered to be of high quality when you’re doing a systematic review meta-analysis,” Dr. Forsyth says. “If we were all amazingly controlled, yes we would be able to increase muscle tissue at this particular time when estrogen’s elevated, but actually proving that in a group of women is sometimes difficult.”

As the research stands today, Yu sees stringent forms of cycle syncing as a form of “putting the cart before the horse”—though understandably so.

“We’re always looking for the thing that can help us kind of hack our training, hack our performance, so it’s really seductive to think about our cycles as this crystal ball,” Yu says. “But from my conversations with various researchers and other experts in the field and looking at the research, the science itself isn’t quite there yet—but that’s not to say there isn’t a benefit from paying attention.”

“It’s really seductive to think about our cycles as this crystal ball.” —journalist Christine Yu

Rather than focusing on proven physiological and performance markers, Devore is encouraged by more subjective research that demonstrates how subjects actually felt about the process, how tuning into their energy and mood affected their fitness, and their desire for more openness about the issue.

Bringing our cycles into the open

Menstrual hormone fluctuations have been ignored, misunderstood, and maligned as long as people with ovaries have menstruated and men have been uncomfortable with that fact.

“It’s 2023 and we still don’t openly talk so much about the menstrual cycle and women’s health; there’s still a lot of secrecy,” Claudia Pastides, MBBS, director of medical accuracy at Flo says. “Whether [the menstrual cycle is] a hundred percent relevant and matters every single day or not, we’ll see. But it’s good at least to know and not to just ignore something so big that happens to [the majority of] women.”

Because tracking your cycle requires paying more attention to your body, doing so may also help some women develop a more meaningful relationship with exercise, perhaps while building a more intuitive approach to movement that prioritizes feeling good. That’s true for everyday exercisers, and even athletes who themselves train on how to tune in.

“Every woman is going to be different, and then also could be different from one cycle to the next,” Dr. Forsyth says. “You have to take ownership of your own feelings, your own perceptions of how your menstrual cycle is triggering things for you, and record that so that that then informs your support team, your coaches.”

But whose business is your period, anyway?

In professional training, both male and female teams collect reams of physiological and training data that informs an athlete’s workout regimen. The inclusion of menstrual cycle data—both objective (like hormone levels and cycle days) as well as subjective (like perceived mood or energy levels)—could be seen as progressive, because it treats the menstrual cycle as just one of many data points that could affect an athlete’s training, and not a taboo mystery that nobody talks about.

However, the push for menstrual cycle data in professional sports does not come without concern. Student athletes in Florida are worried that a move to make submitting menstrual cycle data mandatory is an invasion of privacy, and a smokescreen for transphobic bans against non-binary students’ participation in sports. Dr. Forsyth shares that some players on a women’s rugby team she advises are worried that sharing their menstrual cycle data could affect their playing time.

“They are so concerned about being dropped from the team at an elite level, about sitting on the bench, about losing sponsorship if they say something negative about how they’re feeling,” Dr. Forsyth says. “So there’s still some barriers and there’s still some anxieties about collecting the data and using that to change their training.”

The dangers of ‘feminine fitness’

In September, a new women’s lifestyle and subscription service called 28 launched. It’s a company built around cycle syncing your fitness and your nutrition, promising “Fitness tailored to your body’s natural cycle.” It positions itself as a “by women, for women” service, “empowering women to radically improve their health by embracing their nature.”

What is that “nature”? As discussed in Vice, 28 is an offshoot of the conservative women’s magazine Evie, known for articles promoting transphobia, anti-vaccination misinformation, and contraception fear-mongering. Evie has also eschewed both the “body positive movement” and working out “like men.” So for its own fitness venture, founder Brittany Hugoboom describes the vision as “‘feminine fitness’—the philosophy that you should be exercising and eating according to your cycle.”

When 28 launched, it garnered attention from Vice and others because of its adjacency to the overturning of Roe v. Wade, and the fact that conservative donor Peter Thiel’s Thiel Capital is a major investor. Worries were spiking around what period tracking and other health apps—potentially including 28—might do with our menstrual cycle data in a world where, in much of the country, abortion might not just be illegal, but punishable by law.

In addition to these concerns, Evie and 28 are putting forth an idea of womanhood in which our hormones govern our bodies, sweeping aside the idea that people with ovaries have more going on than what’s happening in those ovaries. Menstrual cycle syncing is supposed to be about empowerment, but at its most extreme, it is about promoting one essentialist ideal of what it means to be a woman over the reality that women can gloriously run marathons while bleeding uterine lining down their legs.

“We are not just childbearing vessels,” Dr. Pastides says. “We can’t deny that hormones play some part, but there’s so much to being female and/or having these hormones than just the hormones themselves.”

Cycle syncing also runs the risk of overstating the impact that hormones may play in our lives and our fitness. “Just because some people might feel awful on one particular day,” says Dr. Forsyth, “that doesn’t matter to some people, they will push through and they will do it anyway.”

Empower, don’t prescribe

Alongside all the potential benefits of breaking taboos and promoting female-forward research, the dangers of the trend come when we vaunt one “natural” way of doing things above others, and allow a tracker to dictate our choices rather than listening to how we actually feel.

“It is absolutely essential research that we need to do,” says Yu. “The caution that I have is around getting too prescriptive about it. At least at this stage, the research doesn’t quite support it. And so if you are hearing about the different recommendations or different training programs, you need to be asking questions.”

Before you embark on a cycle-syncing regimen, Yu suggests looking into who the creators are, digging deeper into the research they’re pulling from, checking out the scientific advisory board, “and just being a critical consumer of whatever it is that’s being sold to you.”

Ultimately, taking your menstrual cycle into account in your workouts should be about helping you feel your best and achieving your goals. Like our cycles and our hormones, what that looks like varies from person to person. And while there are red flags and sinkholes on the cycle-syncing workout road, at least we’re saying the words “menstrual cycle” and “ovulation” without shame—and even contemplating the idea that estrogen could maybe help us get yoked. That’s a version of “feminine fitness” we can get behind.