For decades, Dr. A.C. Hackney struggled to find a succinct way to describe his research to others. “Reproductive physiology” was too vague, and “exercise science” too broad. When people asked about his body of research, his answer usually took more than a few minutes and resulted in a polite-but-dismissive response. It wasn’t until he attended a TED Talk by fellow researcher and friend, Dr. Stacy Sims, that he finally heard the five perfect words that sum up his life’s work:
“Women are not small men.”
The phrase eventually became the central thesis of Roar, Sims’ popular book on female exercise physiology. It’s also the perfect way to summarize and validate Hackney’s 40-plus years of studying women in sport—specifically, how the unique hormonal make-up of men and women causes each group to respond differently to the same training stimulus.
“It’s so clear, and it’s so accurate,” Hackney said of Sims’ motto, with admiration. “The people who have been studying this for years were never quite able to articulate it so well.”
Women are not small men. It may seem like an obvious statement, until you consider the current body of research in sport science. Women are significantly underrepresented in sport and exercise research. Many studies on exercise and physiology use an overwhelmingly (if not exclusively) male study sample. In an analysis of more than 1,300 studies in sport medicine, researchers from the Queensland University of Technology found that an average of 35-37% of study participants were female; only 8% of published studies focus solely on women, and they mostly focus on pregnancy, menopause, or reproductive disease—not sport performance.
It’s a discrepancy that Hackney has been fighting since the late 1970s, when he was a health-biology student at Berea College in Kentucky. The U.S. federal civil rights law Title IX had just been enacted, and with it came a boom in women’s sports participation. As a track and cross-country athlete, Hackney noticed coaches of female athletes were mostly just making it up as they went along, using misguided assumptions and gross inaccuracies about female physiology. This suspicion was confirmed when he befriended, then began dating (and eventually married) a field hockey player named Grace Griffith. As Griffith shared her frustration about coaches who didn’t seem to understand how women worked, pieces of the puzzle started to fall into place for Hackney.
Of course coaches were making it up as they went along; there was no rule book to follow. At the time, most of the research on athletes was conducted by men, on men, and about men. No matter the topic of research, whether on hydration, training adaptations, or injury prevention, it was assumed that women’s bodies would always behave similarly to men’s, but in reduced scale. In other words: Women were just small men.
When Hackney began his research in exercise physiology as a graduate student, he made it a point to study women as a unique demographic, investigating how female sex hormones affect exercise performance. As one of the first researchers to do so, he frequently ran into resistance from fellow researchers and the academic community at large.
“Before, we were always saying, ‘Oh, when women exercise, we should expect to see this, because this is what we saw in men,’ but I was pointing out that we’re seeing women respond differently than what we might expect,” Hackney said. “And I kept hearing, ‘Well, maybe you did something wrong.’ It couldn’t possibly have been that the women were different than men, it had to be that I did something wrong in the context of executing a study that made me get these different results.”
But as Hackney did more studies, it became apparent that women’s physiological responses to exercise were indeed different than men’s. The academic community wasn’t always receptive to this growing body of evidence at first; instead of seeing an opportunity for deeper research, many used a new rationale for keeping the male-centric status quo: “Women are just too complicated to study.”
As a result, most of today’s training, recovery, and nutritional guidelines for female athletes are based on studies done primarily on men. At best, this research gap is keeping female athletes from reaching their potential; at worst, it’s causing irreparable harm to the health and safety of women in sport.
Why Are Women So Hard to Research? (Hint: They’re Not.)
If you ask a dozen researchers why women are underrepresented in scientific studies, you’ll get a dozen different answers.
Some say it’s out of an abundance of caution, as researchers are instructed early and often to treat all women of childbearing age as pregnant, even if they aren’t. This practice began after several failed drug trials of the 1950s – specifically, those involving thalidomide, which caused thousands of children around the world to be born with birth defects, and diethylstilbestrol (DES), which was linked to higher rates of cancer in both mothers and the daughters born to them. In response, the U.S. cracked down on the way research studies (especially those involving drug trials) are conducted. Part of these safety guidelines included excluding all women of reproductive age from most research. Though the National Institutes of Health reversed this policy in 1993, apprehension understandably remains.
But what about women who are not pregnant, because they’re on hormonal contraceptives? Some researchers see that as a complication, too. Hormonal contraceptives—e.g., the pill—influence many aspects of a woman’s physiology, affecting everything from blood pressure to appetite. Researchers prefer clean, simple studies without confounding variables—that way, they can confidently draw conclusions without questions about whether the effect was influenced in any way by outside factors like hormonal birth control.
But then again, women who aren’t on birth control might be seen as the most complicated group of all. Menstruation is not a monolithic experience. Girls experience menarche, or the first menstrual bleeding (i.e., “a period”), anywhere between ages 7 and 18. Their cycles (or duration between periods) may be as short as 21 days or as long as 45 days. These cycles can be disrupted temporarily or permanently by a variety of factors, including pregnancy, perimenopause, menopause, low energy availability, or certain health conditions. During the menstrual cycle, a woman’s hormones—specifically, estrogen, and progesterone—fluctuate. It can be challenging for researchers to tease out whether an effect was caused by a variable they introduced in the experiment or by a biological one that was out of their control.
“So now you have these confounding factors you have to account for with women,” Hackney said. “Are they on oral contraceptives or not? Do they have a regular menstrual cycle? Are they postpartum, perimenopausal, postmenopausal? All of these are factors that can influence your research outcomes. Suddenly, as a researcher, you’re realizing this is a lot of work that you may not want to deal with. So you just study men, because you assume men are easier to study.”
But to say men are a straightforward, monolithic study sample is also an oversimplification. Research suggests that women aren’t the only ones with hormonal cycles that affect physiology. Men’s sex hormones (specifically, testosterone) also fluctuate throughout the day, month, and seasons, though to a lesser degree than women’s. With age, men experience andropause, or a drop in testosterone often referred to as “male menopause.” Testosterone-boosting treatments can also affect a man’s physiology in many of the same ways as hormonal birth control for women. All of these can be confounding factors for researchers, yet men are still often held up as the gold standard for study subjects.
“When they say it’s easier to study men, they’re probably not thinking as carefully as they should,” warns Hackney.
“When you pursue the simplest route in hopes of creating a legitimate study, you can sometimes make the study less legitimate. You can’t just ignore 50% of the population: women."
Actually, Extrapolation Doesn’t Work
When hundreds or even thousands of studies have been done on a topic, it’s easy to conclude that the findings are fact. But, in fact, the facts only ring true for the demographic studied. When researchers take the time to replicate or create studies with women as the focus, they sometimes discover that advice that is helpful for men is actually detrimental for women.
Take, for example, almost all advice given about the ideal “race weight” for endurance athletes. In 1978, Dr. Kirk Cureton and his colleagues at the University of Georgia published a seminal study on body mass and speed, stating that every pound of extra weight slowed runners down an average of 1.4 seconds per mile. This weight-speed study has been replicated time and time again, including a much-publicized conclusion from a 2014 PLOS One study that plotted Body Mass Index (BMI) and speed of runners: “lighter and smaller is better for endurance events.” The tight and tidy quote is often repeated by coaches, athletes, and irresponsible media outlets as a surefire ticket to a faster run split.
But those studies were done primarily on male endurance athletes. In studies that include all endurance athletes, the weight-speed correlation doesn’t always hold. In fact, new studies suggest that for women who are not overweight or obese, weight loss may actually be counterproductive for improving athletic fitness and aerobic performance. When studying women under 30, researchers found that those with the highest aerobic fitness had an average BMI of 23.2— a number found at the upper limit of the World Health Organization’s recommended BMI for good health.
This “race weight” myth has wreaked havoc on female endurance athletes who, for years, have been told by coaches to slim down in order to speed up. Women naturally carry a higher percentage of body fat than men, and trying to override that biological difference in body composition can have harmful consequences. If a woman loses too much weight, she risks losing too much muscle, which ultimately slows her down. If the weight loss continues, a cascade of negative effects can result: irregular or absent menstrual cycles (known as secondary amenorrhea), low bone mineral density, bone fractures, disordered eating, high cholesterol, anemia, blood pressure and heart rate irregularities, depression, anxiety, and the development of eating disorders.
Relative Energy Deficiency in Sport, or RED-S (formerly known as the Female Athlete Triad), is a potentially serious syndrome of menstrual dysfunction, low energy availability, and decreased bone mineral density (among other physical and mental health issues) known to occur frequently in female athletes. This is particularly prevalent among women competing in sports that emphasize a lean body, such as distance running, track, swimming, and triathlon. The prevalence of secondary amenorrhea, for example, is as high as 69% of this athletic demographic, compared with less than 5% of the general population.
In recent years, professional endurance athletes have come forward to share their personal experiences of pursuing the “lighter is faster” ideal. Young running phenom Mary Cain was pressured by coaches to get “thinner, and thinner, and thinner,” ultimately resulting in multiple broken bones, missed periods, and extreme low energy that derailed her performance during what was supposed to be her prime. Elite runners like Tina Muir and Kara Goucher have come forward to share how their training also hindered their ability to become pregnant. Holly Lawrence, the 2016 Ironman 70.3 World Champion, has shared her experience slimming down to an unhealthy weight through unhealthy means, which contributed to injury and poor performance. All of these athletes were simply following what the research supposedly said: “lighter and smaller is better for endurance sports.”
These cases underscore how a man and a woman can follow the exact same advice, same training plan, diet, or injury rehab protocol, but differences in body composition and body function can lead to vastly different outcomes. When researchers actually take the time to study women, they find more and more evidence that, in fact, women are not small men. Hormone fluctuations aren’t a complication, they’re key to understanding and optimizing athletic performance in women.
What We Don’t Know About Female Athletes: A Lot
A woman’s unique physiological makeup influences everything from how she performs in training and racing to how she recovers. As hormone levels change throughout a woman’s cycle, so too does the way she derives energy for exercise, preferring to burn fat for fuel when hormone levels are high and carbohydrate when they are low. Some studies have found hormone levels may affect ligament laxity, suggesting injury risk may increase at various stages of a woman’s cycle. There is also evidence that when hormones fluctuate, so too does a woman’s ability to maintain proper hydration levels, metabolize nutrients, and regulate body temperature—unique factors critical to female athletic performance.
While tracking your period can have performance benefits, there isn’t enough evidence to be able to say for sure what all of this means, much less how to put it into practice.
“A lot of the recommendations, guidelines, principles are based upon the science available when they were developed,” Hackney said. “Regrettably, the science that was available was almost entirely based upon men. I really believe there are honest, good scientists that have not gone in with the intent of ‘I want to exclude women,’ but instead ‘I want to make guidelines, and I have to use the data I have.’ But the data was 99% on men.”
Still, Hackney is hopeful. “Yes, there’s such a void in the amount of good quality research we need. And moving forward, we need many more studies to build a consensus, to conclude convincingly what we’re seeing about women and endurance sport performance. But I also believe someday we’ll reach a critical mass, where we start seeing new recommendations, standards, and guidelines that are more applicable for women.”
A major reason for this optimism is a growing wave of young female researchers entering the exercise and sport science arena. “There is a new explosion of people finally looking at women in the context of exercise, because more and more women are moving into research and science,” Hackney said. “Women are stakeholders now, and they’re saying, ‘Why in the hell haven’t you been studying us?’ I love that. I think that is fantastic. Women are finally shaping the questions and the discussions that we’re having on this research topic, and it’s about damn time.”
OK, So What Now?
One of those women shaping the discussion today is Dr. Ritva Mikkonen of the sports technology unit of the University of Jyväskylä in Vuokatti, Finland. As the lead researcher of the women’s menstrual cycle and endurance training (NaisQs) study, Mikkonen hopes to create guidelines for women that are based on female-specific evidence instead of best guesses. In addition to studying how the menstrual cycle and hormonal contraceptives affect performance in recreational runners, Mikkonen is examining performance solutions designed for women.
“Right now, there’s a big trend in period tracking, or aligning the training cycle with the menstrual cycle. You can buy certain apps that recommend endurance training in a certain part of the menstrual cycle,” Mikkonen said. “But there hasn’t been a single training study that indicates it would be beneficial in any way for the female athlete. The overall concept of these apps tracking the period alongside training is a fantastic idea, but no one has tested whether there are benefits to periodizing endurance training with the menstrual cycle in terms of weight management, fatigue resistance, or improving VO2max.”
Mikkonen doesn’t say this as a criticism, but as a point of curiosity. Women’s participation in endurance sports is at an all-time high, and athletes in running and triathlon want every advantage possible, whether it comes in the form of supershoes or superfoods. A period-tracking app designed to recommend specific workouts on specific days based on where one is in your cycle seems to make perfect sense, given the growing awareness of the unique physiology of women. The market for “femtech,” or women-specific health technology, could be worth as much as $50 billion by 2025. But just because an app claims to be proven or rooted in research doesn’t automatically make it true.
“Nothing is proven yet,” warns Mikkonen. “Apps that recommend strength training during a certain phase of the cycle essentially made that recommendation on just three small studies. In terms of combining strength and endurance, and especially programming training using phases of the menstrual cycle, ‘period periodization’ hasn’t been researched at all.”
"I’m not saying these suggestions are based on bad research, only that there isn’t enough research. In the medical world, we wouldn’t recommend a drug based on such a small body of evidence.”
Her latest study, which collects in-depth physiological, performance, and self-reported data of physically active women over a 16-week period, aims to compile a multitude of factors to tease out the interplay of hormones and adaptations in response to moderate-intensity endurance training and high-intensity interval training. By comparing results of a traditional endurance schedule (where the date of the goal race dictates the timing, length, and intensity of the workouts) with a schedule based on the hormonal fluctuation of the menstrual cycle, it’s hoped the study will yield insights that could one day influence global exercise recommendations for women.
In the interim, Mikkonen says a heightened interest in women’s physiology has created an enthusiasm for her field that would have been unheard of even ten years ago. Women are eager to learn about their own physiology and performance, and researchers are discovering that what was once thought to be “too complicated” is really not. One-third of women in the U.S. already monitor their menstrual cycle through various period-tracking apps, so an extra burden is not placed upon researchers or study subjects to collect data. Female athletes have begun to realize that blanket recommendations for men and women don’t work, and have taken it upon themselves to find out what does. Today, “women are not small men” is no longer a summary of the limited research available about women in sport, but a rallying cry for more.
“More women are listening to their bodies and making decisions for their training based on how they’re feeling, not what people are telling them to do. They’re looking for answers, just like we are,” Mikkonen said.