Rebecca Demorest, a sports-medicine pediatrician, told me that it is common for her to treat young women with injuries from head to toe. “They ache and they hurt and they use pain medicine and try to keep on playing,” she said. “When they finally get to the point they can’t play, they come in to see me. . . . They have a series of nonspecific, overuse injuries that comes down to being worn out. Don’t get me wrong. There’s a chain of events with boys too. But I see it more with the girls.” (I spoke with Demorest when she was based at Children’s Hospital in Washington; she has since moved to the Women’s Sports Medicine Center at the Hospital for Special Surgery in New York.)

Comprehensive statistics on total sports injuries are in short supply. The N.C.A.A. compiles the best numbers, but even these are based on just a sampling of colleges and universities. For younger athletes, the numbers are less specific and less reliable. Some studies have measured sports injuries by emergency-room visits, which usually follow traumatic events like broken bones. A.C.L. and other soft-tissue injuries often do not lead to an E.R. visit; the initial examination typically occurs at the office of a pediatrician or an orthopedic surgeon. Studies of U.S. high-school athletics indicate that, when it comes to raw numbers, boys suffer more sports injuries. But the picture is complicated by football and the fact that boys still represent a greater percentage of high-school athletes.

Girls are more likely to suffer chronic knee pain as well as shinsplints and stress fractures. Some research indicates that they are more prone to ankle sprains, as well as hip and back pain. And for all the justifiable attention paid to concussions among football players, females appear to be more prone to them in sports that the sexes play in common. A study last year by researchers at Ohio State University and Nationwide Children’s Hospital in Columbus, Ohio, reported that high-school girls who play basketball suffer concussions at three times the rate of boys, and that the rate for high-school girls who play soccer is about 1.5 times the rate for boys. According to the N.C.A.A. statistics, women who play soccer suffer concussions at nearly identical rates as male football players. (The research indicates that it takes less force to cause a concussion in girls and young women, perhaps because they have smaller heads and weaker necks.)

But among all the sports injuries that afflict girls and young women, A.C.L. tears, for understandable reasons, get the most attention. No other common orthopedic injury is as debilitating and disruptive in the short term  or as likely to involve serious long-term consequences. And no other injury strikes women at such markedly higher rates or terrifies them as much. Rachel Young, a former soccer player at Virginia Tech who had to stop playing after two A.C.L. ruptures and substantial cartilage damage in her right knee, told me that young women she knew feared the injury but rarely talked about it. “A.C.L. is like a curse word,” she said. “You just cringe when you hear it.”

AN A.C.L. DOES NOT tear so much as it explodes, often during routine athletic maneuvers  landings from jumps, decelerations from sprints  that look innocuous until the athlete crumples to the ground. After the A.C.L. pulls off the femur, it turns into a viscous liquid. The ligament cannot be repaired; it has to be replaced with a graft, which the surgeon usually forms by taking a slice of the patellar tendon below the kneecap or from a hamstring tendon. One reason for the long rehabilitation is that the procedure is really two operations  one at the site of the injury and the other at the donor site, where the tendon is cut.

Janelle suffered her first A.C.L. injury at practice with her club during a routine drill. When she planted her left leg to shoot, the knee buckled. Her mechanics felt no different than they had thousands of times before: Decelerate. Fix on the target. Kick. There were few things in her life she did with more ease or joy. Her second A.C.L. injury occurred the following summer at the annual Texas Shootout in Houston, a prestigious event that attracted 300 teams and 360 college coaches as well as major corporate sponsorship, including Adidas, Gatorade and the Texas Sports Medicine Center. In the first game, she ruptured the A.C.L. in her other knee. “This time I was pretty sure what it was,” she said. “I was chasing after this girl, trying to cut to stop her. And it just went out on me.”

She stayed down on the field, screaming. A trainer came out and tried to calm her, assuring her the pain would subside. But her screams came more from anger than pain. She instantly understood that most of her senior season of high-school soccer would be wiped out and worried that no college coach would want to recruit her. (What she did not realize was that if college coaches shunned girls with a history of serious knee injuries, they would struggle to put quality teams together.)