Sixteen-year-old Phoebe Papademetriou was playing defensive midfield during an intense high-school varsity soccer game last October when she spotted the ball making its way toward her. She shifted her weight to her right leg as she prepared to kick the ball and heard a "pop" followed by a rush of searing pain as her knee collapsed beneath her. An MRI at a local New York hospital confirmed the diagnosis: she had torn her ACL, shorthand for the anterior cruciate ligament, which stabilizes the knee as it rotates. It was a diagnosis with which she was all too familiar. "I know four or five people who have had this happen to them," she says. The day she had surgery, her mother looked around the recovery room and realized every patient was a young woman. When Papademetriou's mom—a friend of ours—asked about it, the nurse seemed unsurprised. "It's ACL day," she replied.

What that nurse knew, which our friend did not, is that ACL injuries are four to eight times more likely to occur in females than males; and, as the number of young women getting into competitive sports has grown, so has the number of ACL injuries. Young women who play sports that demand frequent direction changes, twists, jumps, and sudden stops—such as soccer, basketball, handball and tennis—are especially vulnerable. The American Academy of Orthopaedic Surgeons estimates that about one of every 100 female high-school athletes playing one of these high-risk sports will injure an ACL before she graduates. The risk of injury for college-level players of these sports rises to one in 10. Nationally, about 200,000 people suffer an ACL injury every year.

Dr. Letha Griffin, an Atlanta orthopedic surgeon who specializes in these injuries, says athletes are often pivoting or cutting, landing a jump, or stopping in an awkward way when the injury occurs. It's usually not a contact injury. Doctors don't know exactly why girls are more vulnerable to these tears than boys, but researchers think it's related to differences in anatomy, coordination and stance. Being out of shape increases the chance of these injuries, but even athletes like Papademetriou, who play competitive sports year-round and are in excellent physical shape, clearly are at risk.

Surgeons can repair the ACL and stabilize the knee enough to get athletes back on the field or court within six months. But among those who are still playing sports three years after an ACL injury, about 20 percent will reinjure the same knee or tear the ACL in their other knee. Even more troubling, research indicates that more than 90 percent of those who have successful ACL surgery will experience degenerative arthritis in the injured knee within 10 years, says Griffin. "Typically, they come back in their early 30s because of increased pain in the knee and significant loss of function," Griffin says. "By the time I see them they've given up running, they can't play tennis. Sometimes they can't even keep up with their kids." Doctors use a variety of anti-inflammatory drugs to keep their patients as functional as possible, for as long as possible, because they don't want to do knee replacement surgery until patients are in their 50s or 60s. "Joint replacements only last about 15 years," says Griffin, "and the second joint replacement isn't as good as the first." While researchers search for better surgical procedures, Griffin says "clearly, this is an issue we have to attack with prevention."

What's the best way to do that? Researchers say they've had the most success by training girls to mimic some things that boy athletes seem to do naturally. Dr. Frank Noyes, one of the nation's top knee specialists, says that when male athletes play sports they instinctively keep their knees and hips flexed and their upper bodies equally balanced over their lower extremities. These positions help protect them against ACL injury, he says. Female athletes tend to maintain a more upright position and extend their legs farther out from their bodies while playing. When they jump they often land in an upright position, and are less likely to land on both feet. These actions make them more vulnerable to ACL injuries, Noyes says.

There's also the matter of what Noyes calls the "wiggle wobble" in women's knees, something he first noticed while working with athletes at the University of Cincinnati. "When a woman jumps, her knees tend to wiggle or wobble one way or the other," he says. To counteract that, he started putting together a jumping workout to strengthen the female knee. "We slowed it down, so that female athletes learned to jump under control, kind of like you do in hopscotch," he says. "We started by teaching them to jump on both feet, then added twisting and turning in the air." The training program he developed has been tested in nine Cincinnati high schools. All reduced their female ACL injury level to that of male students; there was no change in the control group of nine Cincinnati high schools that didn't use it.