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The Gender Gap

Female athletes have reached the pinnacles of success in sports. But science on sports performance among women and girls has been slow to catch up.
By Christine Yu


There have been many times when Emily Kraus, MD, sports medicine physician and director of the Stanford Female Athlete Program at Stanford University in California, felt frustrated on behalf of her patients, especially her female athletes. As an endurance athlete herself, Kraus understands that women face different physiologic demands compared to their male counterparts. Yet, health care providers don’t always consider this nuanced context during their consultations and often miss the mark when it comes to interpreting complaints from active women. 

For example, if a woman’s iron levels are on the low end of the normal range, she may be told her levels are healthy. But that baseline doesn’t quite fit someone who’s an athlete, where subclinical abnormalities can negatively affect performance and training. That woman might leave with a blanket prescription for training, nutrition and recovery, feeling like her concerns were misunderstood or dismissed.

Kraus knows there are numerous opportunities to dive deeper into areas related to a woman’s physiology, like her menstrual cycle, and to explore how these aspects influence training, performance and health. “We’re overlooking a lot of more important details underneath the hood,” she says.

The problem? Experts say there are huge gaps in the understanding of female physiology within the context of physical activity and sports, and there isn’t enough evidence-based research to provide concrete recommendations. 

It’s a peculiar gap. On the one hand, the number of girls and women participating at all levels of sports—from physical activity to professional sports—has boomed over the past 40 years. There are over 3.4 million opportunities for girls to participate in high school sports. Opportunities for female athletes in the National Collegiate Athletic Association have increased 291% since the early 1980s. At the global level, women will represent 48% of total athletes expected to compete at the Tokyo Olympics, the highest percentage of female athletes during the modern Olympic era. 

Despite this growing interest and involvement in women’s sport, scientific research hasn’t kept pace, leaving women underrepresented and understudied in the fields of exercise physiology and sports science. That means the current model for athletic training, performance, injury prevention and return-to-sport is based on evidence found in men and then broadly applied to women. This ignores important physiological and biological differences between men and women.

Kraus is part of a growing number of practitioners and scientists dedicated to closing the research gap. “I want to be an advocate for female athletes through doing research and then eventually translating that research so we can be better clinicians, better coaches and even better parents of female athletes,” she says. By specifically studying women, researchers may uncover strategies to maximize performance, reduce injury and help girls and women get into and stay in sports in a healthy way.

Leaving Women Out of Scientific Research

It’s no surprise that the traditional paradigm of biomedical research is based on men. Men have long dominated the fields of medicine and scientific research, largely dictating research priorities. Male bodies have been seen as the gold standard while women have been considered deficient in comparison, a belief dating back to the 19th century. There have also been concerns about conducting research on women, particularly after the sedative thalidomide was found to cause birth defects when taken by pregnant women in the 1960s. Afterward, the National Research Act of 1974 prohibited pregnant women from participating in research studies. 

Sex can be a confounding variable. In particular, the menstrual cycle has complicated the research process. Fluctuating sex steroid hormones can cause physiological variability and create noise in the data that’s burdensome and expensive to account for. Researchers have to run additional tests to determine a woman’s menstrual phase throughout the research timeline. In the past, it was easier to eliminate the female sex from the research process across cell, animal and human studies. Even if women were used in studies, the data weren’t always reported or analyzed by sex. 

Despite the growing interest and involvement in women’s sport, scientific research hasn’t kept pace, leaving women underrepresented and understudied in the fields of exercise physiology and sports science.

The problem is that women are not men. “We cannot extrapolate the data that we collected in men to women and treat them the same way,” says Lacy Alexander, PhD, professor of kinesiology at Penn State University in State College, Pennsylvania, and past chair of the APS Exercise & Environmental Physiology Section. Alexander is also an APS Councilor. “We have to figure out appropriate therapeutics that make sense in terms of the physiology,” she says. 

Take prescription drugs, for example. Women metabolize medication at a different rate than men, but because dosing guidelines are universal, women may experience more side effects. When women took the sleep aid zolpidem (prescribed as Ambien), they reported symptoms such as cognitive impairment, sleep walking and even “sleep driving.” Only then did the U.S. Food and Drug Administration issue sex-specific dosing guidelines for the medication in 2013, approximately 20 years after the drug was first approved.

Even the 2017 hypertension guidelines from the American College of Cardiology and American Heart Association are based on research data derived primarily from men. Men comprised 70% of the study participants that informed the guidelines, and the evidence doesn’t hold up in women, says Jane Reckelhoff, PhD, FAPS, professor and chair of the Department of Cell and Molecular Biology at the University of Mississippi Medical Center in Jackson and past president of APS. Despite the fact that women make up half of the world’s population, hypertension recommendations for women are included in a separate section titled “Special Patient Groups.” 

The scientific community is playing catch-up when it comes to recognizing the importance of sex differences. It wasn’t until 1993 that scientists were required to include women and minorities in clinical trials funded by the National Institutes of Health (NIH). In 2016, NIH enacted a new policy requiring the inclusion of sex as a biological variable in the design, recruitment, analysis and reporting of cell, animal and human research.

Sex Differences and Athletic Performance

Anthony C. Hackney, PhD, professor of exercise physiology and nutrition at the University of North Carolina at Chapel Hill, realized over 40 years ago that female athletes weren’t studied to the same extent as male athletes. With his interest in endocrinology, he understood that sex steroid hormones coordinate more than just reproductive function. They also have important roles related to metabolism, thermoregulation and hydration. He wondered: Could reproductive hormones—that change across the menstrual cycle and across a lifetime—affect a woman’s potential to exercise and perform?

Hackney was an outlier. Most researchers didn’t ask the question and weren’t very interested in studying sex differences. According to a 2014 study in the European Journal of Sport Science, women comprised only 39% of total participants in studies published in three leading sports science journals, and fewer than 14% of studies investigated women only. What’s more, a 2021 study in the Journal of Science and Medicine in Sport found that between 2000 and 2020, women represented just under 25% of first authors of randomized control trials in 14 of the top sports science journals. Women also occupied only 18.3% of editorial board positions, which could, in part, account for why female-related topics aren’t prioritized. 

As a result, there are few studies that specifically examine why differences may occur between female and male athletes or explain what that means for female athletes. According to Kraus, without data, the sports science and sports medicine community can’t come up with evidence-based treatment and management protocols to help women prevent injury and achieve their athletic and personal potential. In failing to interrogate these questions, Hackney says we leave unexplored the nuances in female physiology and miss opportunities to improve women’s health and training and lower women’s risk of injury. And at the elite levels, where races and medals are won by slim margins, those nuances could mean the difference between first and fourth place. 

Helping Female Athletes Excel

In recent years, there has been growing recognition that women and men experience exercise and sport differently and that women could benefit from sex-specific guidelines based on their physiology and lived experience. For example, studies indicate that women are two to eight times more likely to rupture their anterior cruciate ligament (ACL) compared to men and are more likely to retear their ligament. Women also return to sport at a lower rate after an ACL injury. In sports such as soccer, where men and women play with the same rules and equipment, women experience concussion at a higher rate. They also report a greater number and more severe symptoms with brain injury, particularly vestibular and ocular symptoms, and take longer to recover and return to play, school or work. Even how female and male athletes perform in hot conditions can differ due to differences in sweat rate and body fluid regulation. 

In particular, researchers have turned their attention to the menstrual cycle to understand how the dynamically fluctuating hormonal environment influences areas such as performance, chronic training adaptations, injury, endurance and strength. Plus, most people who menstruate acknowledge that their cycle and its accompanying symptoms—such as cramps, bloating, fatigue, nausea—affect their fitness and how they feel when active. Understanding these changes can provide opportunities for female athletes to optimize their performance potential while mitigating symptoms they may experience. 

While the jury’s still out on the definitive impact of the menstrual cycle and hormonal contraceptives on exercise performance, it has opened a larger conversation about female-specific factors in sport. For example, in the lead-up to its historic fourth World Cup title in 2019, the members of the U.S. Women’s National Soccer Team tracked their menstrual cycle and symptoms. It was another piece of the performance puzzle for the team, just like injury prevention or nutrition. Players became more attuned to their bodies. Based on this information, the team’s high-performance staff helped players manage their anticipated symptoms with nutritional interventions, recovery strategies or even just reminders to get more sleep. 

It’s Not Just about Female Athletes

Kraus points out that studying sex difference doesn’t just benefit women; it can benefit all humans. “You can flip the research the other way too and understand male athletes better,” she says. Take research on the female athlete triad, for example. The triad describes the interrelationship between energy availability, menstrual function and bone mineral density, where inadequate nutrition can lead to menstrual cycle dysfunction and low bone mineral density and put a woman’s health at risk. While the triad was first noted in female athletes in the 1980s and 1990s, research and discussion in this area opened the door to consider similar hormonal disruptions in male athletes, Kraus says. It has led to the identification of new research areas such as the male athlete triad and relative energy deficiency in sports. 

Alexander says that learning more about biology and physiology can also shed light on non-binary and transgender athletes. “What are the health implications of hormone therapy? What do we need to think about? What do we need to do from a study design perspective?” she says. “It’s really important. We need to address these things.”

Still, there’s a long way to go in terms of building an evidence base of quality research on female athletes. But Alexander is optimistic and excited for the future. “There are a lot of really great women and men scientists who are examining sex as a biological variable,” she says. “I’m excited for what we’re going to find and how understanding these differences is going to benefit the public."

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“We cannot extrapolate the data that we collected in men to women and treat them the same way. We have to figure out appropriate therapeutics that make sense in terms of the physiology.”Lacy Alexander, PhD