The third time around, Megan Rapinoe’s reaction to a potentially career-ending knee injury went no further than an eye roll. She had torn her anterior cruciate ligament. She could reel off the recovery schedule from the top of her head. She could see, crystal clear, the next nine to 12 months spooling out in front of her.

The surgery, the painstaking rehab, the grueling weeks in the gym, the anxious first steps on the turf, the slow journey back to what she had once been. As she considered it in 2015, she felt something closer to exasperation than to despair. “I was like, ‘I don’t have time for this,’” she said.

The first time had been different. She had torn the anterior cruciate ligament in her left knee at age 21, when she was a breakout star in her sophomore year at the University of Portland. At that time, she felt what she called “the fear” — the worry that it might all be over before it had begun.

A year later, she had done it again: same ligament, same knee, same arduous road back. It did not stop her from doing all that she had dreamed of doing. She turned pro. She was named to an all-star team. She represented her country. She won a gold medal at the Olympics. She moved to France. She played in two World Cups. She won one of them.

And then, during a training session in Hawaii in December 2015, months after her 30th birthday, it happened again. This time, it was the right knee, and this time, her reaction was different. “It changed for me as I got older,” she said. “That one was like an eye roll. ‘This is annoying. I know what it is going to take to come back’. But generally, I think there’s this fear. Is this going to be the end? Am I going to come back from this? Am I going to have pain forever?”

Over the last year or so, that fear — and the searching questions it prompts — has coursed through women’s soccer. The sport has at times seemed to be in the grip of an epidemic of A.C.L. injuries, one so widespread that at one point it had sidelined a quarter of the nominees for last year’s Ballon d’Or.

Alexia Putellas, the Spain midfielder who won that award and the consensus pick as the best player of her generation, has recovered in time to grace the World Cup, the sport’s showpiece event. But countless other stars have not. They will, instead, spend their summer at home, nursing their injuries, cursing their luck.

The list is a long one. Catarina Macario, the U.S. forward, tore the A.C.L. in her left knee last year and could not regain her fitness in time. She will not be present in Australia and New Zealand. Nor will two of the stars of the England team that is hoping to dethrone the United States: The team’s captain, Leah Williamson, and its most productive goal-scorer, Beth Mead, both fell victim to A.C.L. injuries this season.

The Olympic champion, Canada, has lost Janine Beckie. France has not been able to call upon Marie-Antoinette Katoto or Delphine Cascarino. The Netherlands, a finalist in 2019, is without striker Vivianne Miedema.

But these are just the famous names, the familiar faces, the notable absentees. The problem has become so acute that, at times, it has strained tensions between national teams and the clubs that employ the players from which their rosters are drawn, with at least one high profile European coach suggesting that too much was being asked of the athletes.

Miedema herself pointed out that, this season alone, almost 60 players in Europe’s five major leagues had torn their A.C.L.s. “It is ridiculous,” she said earlier this year. “Something needs to be done.”

Working out precisely what that might be, though, is more complicated than anyone would like.

Lack of Knowledge

There is fear, of course, for players who are enduring those long weeks of recovery, but it is not the only type of fear. In Europe particularly, over the last 12 months, the sheer scale of the issue — the numbers of players being struck down by torn A.C.L.s — set off a psychological contagion.

A number of national associations, as well as local offices of FIFPro, the global players’ union, reported inquiries from active players — those who had seen teammates or opponents or friends condemned to months on the sideline — seeking reassurance, solace or even just basic information.

“The players are asking for research,” said Alex Culvin, FIFPro’s head of strategy and research in women’s soccer. “We’ve had a lot of feedback from players saying they feel unsafe. You saw it last season — at times, players were not going in for tackles as they normally would because they were worried about injury.”

The problem, Culvin said, is there is not enough research available for anyone to give the players clear answers. European soccer’s governing body, UEFA, has been running an injury surveillance study on men’s soccer, for example, for more than two decades. The women’s equivalent has been operating for only five years. “That lack of knowledge creates fear,” Culvin said.

It is established fact that women are more at risk of suffering an A.C.L. injury than men. Quite how much more at risk is a little murkier. Martin Hagglund, a professor of physiotherapy at the University of Linkoping in Sweden, puts the risk at “two to three times greater, based on a systematic review of studies.” Culvin goes a little higher: Some studies, she said, suggest the risk for women could be “six or seven” times as great as that for men. “There is a real range,” she said.

The issue of why that might be is more contested still. Traditionally, much of the research has focused on biology. “There are obvious anatomical differences” between men’s and women’s knees, Hagglund said. Not just the knees, in fact — the whole leg. Some studies have suggested that women’s A.C.L.s are smaller. There are differences in the hips, the pelvis, the engineering of the foot.

Increasingly, too, there is a body of evidence to suggest there is a link between hormonal fluctuations and susceptibility to injuries in general, and A.C.L. injuries in particular. Chelsea, one of the leading clubs in England’s Women’s Super League, now tailors players’ training loads at specific phases of the menstrual cycle in a bid to mitigate the impact.

As a paper published in the British Journal of Sports Medicine in September 2021 pointed out, though, the instinct to focus purely on physiological explanations is both rooted in and serves to reinforce the misogynistic stereotype that “women’s sport participation is dangerous predominantly due to female biology.”

It also runs the risk, in Hagglund’s mind, of turning a blind eye to the host of other issues that may have played a part in depriving the World Cup of so many of its brightest lights this month. “The focus on anatomical differences means we have left out the other parts, the extrinsic factors,” he said. It just so happens that those are the ones that might, feasibly, be addressed.

Injury as a Measure of Value

It is perhaps natural that for the players themselves, the cause of the run of A.C.L. tears is obvious. “We keep adding games left, right and center,” said Miedema, one of four players at Arsenal alone who have sustained the injury this season. “Instead of 30 games a season, we now play 60. But we don’t have the time and investment that is needed to keep players fit.”

Kristie Mewis, a U.S. midfielder, contended that the “intensity” with which women’s soccer is now played had compounded that effect. It is not just that there are more games, she said. It is that they are exponentially faster, more physical and more demanding than ever before. “As the game is growing, it’s getting more competitive,” she said. “Maybe stress has something to do with it.”

Rapinoe would endorse both ideas — “the load and intensity are different,” she said — and would add that while women’s soccer has professionalized on the field at breakneck speed, it has not always matched that pace off it.

“We don’t generally charter; we don’t fly private,” she said. “We don’t have the resources. So with recovery, you’re being asked to produce a bigger load than you ever have but with less resources than you really need to do that.”

To Hagglund, that is only the start of a long list of possible structural, cultural factors that might be at play. “Women’s soccer does not have the same organizational support as men’s,” he said. That applies not just to travel, but to the number and the quality of medical staff members, physiotherapists, nutritionists.

Likewise, young female players, until relatively recently, did not have the benefits of the same sort of specialized strength and conditioning training that is commonplace in boys’ academies. Women’s teams have what he called smaller “competitive” squads — they rely heavily on a handful of high-profile players, ones who cannot afford to be rested. “That means they are more exposed to fixture congestion, there is less rotation, they are more likely to play with an injury,” he said.

And then there are the environmental problems. Women’s teams do not play on the same perfectly manicured lawns that top men’s teams do. “In Scandinavia, certainly, it is still quite common for teams to play on artificial turf,” he said. The players must do so, often, while wearing shoes designed with men’s feet, rather than women’s, in mind.

As diffuse as all of those problems are, they come down to much the same thing in Culvin’s mind. “It is a question of value,” she said. “What value do we place on an athlete? The players might be professional, but the conditions around them are not always suitable for professional athletes. There is not equity in the workplace until we value them properly in all components — the fields, the stadiums, the support staff around them.”

The Right Fit

Laura Youngson is always surprised, even now, by the number of players she encounters who have convinced themselves that soccer cleats are designed to be uncomfortable. “That’s the perception,” she said. “That they’re supposed to feel like that, and that women, in particular, are just supposed to put up with it. They’re really not meant to be like that.”

Still, the belief is widespread. Earlier this year, an in-depth study conducted by the European Club Association and St. Mary’s University, London, found that 82 percent of elite female players experienced “pain or discomfort” from the footwear they wore while playing.

The reason for that is simple. In contrast to running, say, where major footwear brands realized long ago that women and men required — and would buy — different types of shoes, the soccer versions sold to women are, largely, not actually designed for them. The abiding market principle has effectively been, as Youngson put it, “that women are just small men.”

For a long time, like everyone else, Youngson just accepted that her soccer shoes never seemed to fit quite right. Then, after organizing a charity game on Mount Kilimanjaro in 2017, she realized that she was not alone. Even the professional players on the trip had the same complaint. She saw an opportunity — both a business one and a moral one — to put it right.

Since then, the company she founded, Ida Sports, has conducted extensive research to produce the first custom-made women’s soccer cleats. They found that women tended to have narrower heels, wider toe areas and higher arches. (They are also more likely to change than men’s are, particularly during and after pregnancy.) That means they “interact differently with the ground,” something that Ida Sports has tried to remedy by redesigning the sole of the shoes she makes.

There is also enough evidence to suggest that the shape and structure of women’s feet may make them more susceptible to injuries, both chronic and acute, including A.C.L. tears. Youngson does not claim to have a silver bullet for the knee injury epidemic, nor does she believe that wearing better-fitting shoes will end the problem on its own.

“But there is definitely an opportunity for further research,” she said. “People are doing great work studying hormones and behavior and other things. We know boots and surfaces. There are definitely recommendations that we would make. The issue is, how do we keep more players on the pitch? Even if it is for a 1 percent gain, it is worth it.”

Like Rapinoe, the former England international Claire Rafferty endured three A.C.L. injuries in her career. As with Rapinoe, her reaction changed over time. After her first, in her left knee, she felt “invincible,” as if she had gotten her bad luck out of the way early. She was only 16. It would, she assumed, be smooth sailing from there.

She did not know then that the single greatest risk factor for sustaining an A.C.L. injury is having experienced one. Research suggests that 40 percent of players who have torn a cruciate ligament will do so again — in either knee — within five years. It is closer, in other words, to the flip of a coin than a roll of the dice.

Rafferty learned that the hard way. In 2011, she tore the A.C.L. in her right knee. That time, she recalls being “in shock.” She did what she could to mitigate the risk. Despite her entreaties, her coach at Chelsea, Emma Hayes, regularly refused to allow her to play on artificial surfaces. Two years later, Rafferty tore the A.C.L. in her right knee again.

“Nobody thought you could come back from three A.C.L.s then,” she said. Rafferty did. Physically, at least. Mentally, the scars did not heal. “I wasn’t calm,” she said. “I thought every game could be my last. I was playing with a lot of fear. I had quite a lot of anxiety. I couldn’t play like I did before.

“I remember hearing people ask, ‘What’s happened to Claire Rafferty?’ I wanted to tell them that I couldn’t run properly because I was so afraid. I didn’t enjoy playing football. I started to resent it.”

That fear, the one felt by the players missing this year’s World Cup, the one shared by all those who now feel unsafe on the field, had overwhelmed and inhibited her. She knew what she had to do. Long before her career should have ended, she walked away. She was 30. For women’s soccer, the real risk of its A.C.L. epidemic, the one rooted in lack of knowledge and a historical lack of care, is that she will not be the last.

Jeré Longman and Claire Fahy contributed reporting.