Why are women more likely to get ACL injuries?
Sports scientists are trying to figure out why female athletes are so prone to a specific type of knee injury.
With six minutes remaining in the third quarter of her college basketball game, Liz Kitley, a then-center at Virginia Tech, jumped for a left-handed layup to tie the score. She made the shot but instantly fell, holding her left knee under the net. “I knew exactly what that really weird feeling was,” says Kitley, a three-time Atlantic Coast Conference player of the year and an all-American, who now works at Virginia Tech as an Assistant to the Head Coach. In high school, Kitley had torn her right Anterior Cruciate Ligament (ACL), and she knew right away that, this time, she’d torn her left.
Many elite female athletes—like Kitley, USC’s JuJu Watkins, Cameron Brink of the LA Sparks, and most recently Olympic skier Lindsey Vonn—have torn their ACLs. In fact, ACL tears are common among female athletes of all ages. Girls and women face between two to eight times’ higher risk of tearing their ACLs compared with their male counterparts when playing similar sports—and multisport female athletes have close to a 10 percent risk of tearing their ACLs in high school or college. We’ve recognized this increased risk to girls and women since the 1990s. “It’s something that everyone's kind of aware of,” says Kitley, “but nobody really knows why.”
Experts point to several potential explanations: anatomical aspects of the female body, such as a wider pelvis and an increased Q angle, the angle between the quadriceps muscle and the patella tendon; genetics; and biomechanical movements or motor control, “so how we move our body in space,” says Nina Freitas, a senior physical therapist at the University of California San Francisco (UCSF) Women’s Sports Center, who adds that “strength differences” in the hamstring and quad and “hormonal factors” may also play a role in the disparity.
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Are periods a high-risk time for players?
Indeed, the role of hormones, specifically menstruation, has been suggested since the nineties. “The thing I can’t get over is when I tore my ACL in 1998,” says basketball legend Sue Bird on her podcast, A Touch More, last year, “the very first question the doctor asked me when I saw the surgeon: ‘Are you on your period?’ They’ve known there might be a connection here.” On the same podcast, retired soccer star and co-host Megan Rapinoe said she’d had three ACL tears during her period and Kitley remembers being on her period for her first ACL tear and on birth control “sugar pills” for the second. Despite these stories, the evidence for a connection remains inconclusive, says Georgia Brown, a sports scientist for the San Diego Wave FC.
In 2025, a FIFA-funded study at Kingston University was launched to focus on female ACL tears and the menstrual cycle. The study, led by PhD student Blake Rivers, will track participants through four menstrual cycles, monitoring hormonal levels while conducting biomechanical testing that includes strength testing and 3D and electrographic analysis. “We are kind of looking at all of the main movements that we think relate to the largest injury risk without exposing them to too much,” says Rivers. “The better we understand the interaction and what is happening biomechanically during these key hormonal windows, the better we can quantify that level of risk within the interaction.”
The two phases that are considered the riskiest, according to Rivers, are the two with the most significant hormonal changes: the late follicular ovulatory period and the mid-luteal phase. He notes that these phases are believed to be the riskiest as estrogen, which impacts ligament laxity, increases just before ovulation, and progesterone, which “can have an effect on neuromuscular control and inhibition,” surges in the mid-luteal phase. Rivers, co-lead James Brouner and team hope to publish the results in late 2026.
Meanwhile, research published in 2024 by Emily Parker, a Physical Medicine & Rehabilitation (PM&R) resident doctor at MetroHealth, also suggests that the highest risk is in the mid-luteal phase. Parker knows this topic on a personal level: she tore her left ACL in high school and then re-tore it six months later. (Luckily, she healed well enough to later play Division 1 basketball.) Parker’s scholarly work focused on relaxin, a menstrual cycle hormone mostly commonly associated with childbirth, as the primary culprit. “Relaxin initiates menstruation by suppressing and breaking down collagen in the tissue around its receptors in the uterine lining,” says Parker via email. “These receptors are also found on women’s ACLs, raising concerns about the luteal phase and women’s knee-injury risk. Estrogen amplifies the impact of relaxin by priming receptors in relaxin-sensitive tissues to be ready to respond.”
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A serious injury
The lack of clarity surrounding female athletes and ACL tears is unsurprising given that only six to nine percent of sports science studies have focused on female athletes, according to a study published last year. Luckily that trend seems to be shifting. “We have this big push of women’s sports,” Freitas says. In 2024, Nike and FIFPRO launched Project ACL, a three-year study to better understand ACL injuries in female athletes. And last year, UC Berkeley announced a new women’s health initiative that will center research on women athletes, including on ACL injuries.
It's high time that ACL tears in girls are women are taken more seriously. An ACL tear can wreak havoc on girl’s and women’s bodies and minds and can be potentially career-ending for elite athletes. When the ACL is completely ruptured, the “gold standard” is surgery, according to Dr. Sara Edwards, an orthopedic surgeon at UCSF’s Women’s Sports Center, usually followed by nine to twelve months of extensive physical therapy. Females are at a higher risk of re-injury and it “can become a lifelong issue,” Edwards says. Kitley remembers, “it's such a long process. . . For me, I missed my last NCAA tournament, and I missed my ACC tournament and then had to be on crutches at the WNBA draft and then couldn't walk for a while. It completely consumes your life and still dealing with new stuff almost two years later is really frustrating.”
The uneven playing field
While experts wait for more conclusive information, female athletes can take measures to protect their ACLs. Programs that include plyometric exercises, like the FIFA 11+, reduce risk of ACL tears up to 60 percent. “The biggest barrier is getting coaches to actually do the warm-up or getting teams to do the warm-up,” says Freitas. Prevention programs can start as early as six years old, Edwards says, noting the importance of educating coaches, parents and athletes. And prevention, of course, isn’t confined to girls. Most of Edwards’ patients are 40 years and older.
Ultimately, “there isn’t one answer,” why females are at a higher risk, says Joanne Parsons, an associate professor in the Department of Physical Therapy at the University of Manitoba. She and her colleagues Sheree Bekker and Stephanie Coen propose that social differences in the treatment of male vs. female athletes might contribute to the increased rate of ACL tears faced by women and girls. It’s “not about discounting biological factors,” says Coen. “It’s opening up to considering an array of other factors that may matter.”
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For example, there are known inequalities in how both girls and women are exposed to play and sports; “a lifetime of gendered exposures that have set up uneven conditions for risk,” says Coen. Girls and women are often introduced to sports and training later in life, play at unfavorable field times or on fields or courts of lower qualities than the men, and don't use equipment like cleats designed for female bodies. Many female professional athletes work more than one job, which leads to muscle fatigue and also complicates recovery before and after a tear. (According to a report by FIFPRO, over a quarter of elite female soccer players had a second job and it’s estimated that professional female soccer players globally on average earn $10,900 a year.)
Many female athletes, young and old, are also not encouraged to lift weights in the same way their male counterparts are, which is important for prevention, notes Edwards, Freitas and Parker. “My 14-year-old female athletes that I see in the clinic, many times, I’m one of the first providers talking to them about a strength program whereas many of the 14-year-old male athletes. . . .are with their buddies already experimenting with weightlifting or have had exposure to the gym in a different way,” says Freitas. Fortunately, unlike anatomical differences, Parsons and Coen note that many of these gender environmental differences are modifiable.
The purpose of this increased knowledge, of course, is not to hinder girls and women but to continue to empower them. “I don’t want it to ever be conveyed that women should stay away from more physical sports,” says Parker, adding, “It’s more just we need to have better knowledge, quite honestly, of basic biology that’s affecting women’s joint health so that we can keep them healthy while they play.”
