College women take center stage tonight, playing a brand of basketball that did not exist two decades ago. The stationary set shot, once a staple of women's basketball, has been left behind by a game that emphasizes finesse, aggression and unrelenting speed.

The NCAA championship game in Kansas City showcases perhaps the most rapid and remarkable quality upgrade in the history of women's sports. That upgrade has come, however, with an awful price tag. Season-ending knee injuries for women playing college basketball are four to six times more frequent than for men, according to a number of recent studies.

For Nygaard, her knee injury three weeks ago was double agony. As a freshman four years earlier, she tore the ACL in her other knee during an exhibition game. She recovered from that injury with the help of weight training, which she said has made her "legs as strong as they have ever been."

That strength, though, was not enough. Her most recent knee injury occurred when she grabbed a rebound, dribbled to the sideline and, coming down off a shot, awkwardly planted her left foot.

"The trainer on the bench heard it pop. It was much more painful than the first [ACL tear]. It feels like your leg getting cut off. I laid there and screamed and was pounding my head on the floor. They took me off the court. It was the end of my world, playing basketball at Stanford is my world," said a tearful Nygaard.

For a cluster of reasons that orthopedic researchers are struggling to piece together, female athletes -- including the most highly conditioned ones and especially basketball players -- are more susceptible than men to tears of the anterior cruciate ligament. The ACL runs through the center of the knee joint, controlling its pivoting motion and working with major thigh muscles to stabilize the knee. Recovery from an ACL tear, which is surgically repaired using ligament grafts, usually takes between six months and a year.

While the risk of ACL injury for female basketball players is clearly higher than for men, it should not be overstated. Considering the hundreds of thousands of women playing sports, orthopedic researchers say the chances that any one female athlete will have an ACL tear are low. Once every three seasons there will be a member of a women's basketball team that will suffer an ACL tear, according to National Collegiate Athletic Association data. For a men's basketball team, it's once every 11 seasons.

Since the 1972 passage of Title IX, which ensured equity for women's sports at schools getting federal money and which opened the door for an unprecedented increase in women's participation in college and high school athletics, knee injuries in basketball (and to a lesser extent in soccer) have emerged as a glaring and excruciating exception to a general pattern of gender equality among injuries in sports.

A study of NCAA basketball players between 1989 and 1993 found women to be four times more likely than men to have ACL injuries. A New Jersey study last year found that college women playing basketball between 1994 and 1996 were six times more likely than men to tear their ACLs.

The study of 11,780 New Jersey high school and college players also showed that women increased their risk of season-ending injury threefold when they moved from high school to the faster-paced college game. There was no increase in knee injuries for men moving from high school to college games.

When an ACL is torn, it "feels like there is a string inside your knee and then that string pops while it is twisting and is the worst feeling you can ever feel," said Nicole Jones, 17, an All-Met and all-state center at Madison High School in Vienna, Va., who injured her right knee when she was a high school freshman.

Before the 6-foot-2 Jones tore her ACL, she said she had "no clue" that female basketball players were at any special risk of knee injury.

"In high school, coaches don't talk to players and say that this is something that could happen. Even my teammates now, they don't understand where the ACL is," said Jones, who recovered after surgery with aggressive physical therapy and who will be attending Virginia Tech next fall on a basketball scholarship.

The cause of the higher ACL injury rates in women is far from being understood. For the past five years, research projects across the United States have been trying to tease out the most important factors.

"We just don't know for sure. There is probably not going to be one factor to explain it," said Ed Wojtys, an orthopedic surgeon and team physician at the University of Michigan.

In general, researchers have focused on two areas: the strength of a woman's ACL compared with that of a man's and the different ways that a woman might put stress on the ligament compared with a man.

There is a clear gender difference in the way men and women injure their ACLs. Men usually do it in contact injuries, smashing into each other. Women usually tear the ligament without contact, while cutting or jumping.

Some biomechanical research suggests that women, on average, have a smaller and weaker ACL than men of equal height. The size of a woman's ACL and of the bone notch through which it passes is the single most useful tool in predicting her chances of tearing the ligament, said Don Shelbourne, an orthopedic surgeon who has conducted research in Indianapolis.

Shelbourne, who is the team physician for the Indianapolis Colts and whose surgical practice consists primarily of repairing ACL injuries in young female athletes, said that if a woman has an especially thin ACL and narrow bone notch, she might consider sports such as swimming, track or cross-country rather than basketball or soccer. X-rays and magnetic resonance imaging can assess the size of the bone notch and the ACL.

Many other researchers question Shelbourne's insistence that size of the ACL is the critical factor in injury rates among women. They are looking at whether hormonal changes through the course of a woman's menstrual cycle might weaken the ACL or make it more elastic, either of which could increase chances of a tear.

At the University of Michigan and UCLA, researchers have found higher than expected rates of ACL injury shortly before menstruation, when estrogen levels spike. At UCLA, the tissue of the ACL has been found to have receptors that react to estrogen.

"High levels of estrogen basically weaken the tissue and that is probably when the injury occurs," said Stephen Lui, an assistant professor in the department of orthopedic surgery at the UCLA School of Medicine. "We have done research that has not yet been published that will show that hormones indeed contribute to this kind of injury."

As for the differences between how men and women put stress on their knees, some research shows that young women, unlike young men, tend to develop the muscles in their thighs in an unbalanced way, with muscles in the front of the thigh much stronger than those in the back.

Orthopedists believe that the large muscle at the back of the thigh, the hamstring, is essential in stabilizing the knee during and after a jump.

"Women tend to use their quadriceps [muscles at the front of the thigh] more than men and they have a harder time getting peak strength in the hamstring, which we think is the important protective mechanism for ACL," said Wojtys at the University of Michigan.

The increased intensity of women's basketball in recent years has exacerbated the injury potential in skeletal differences between women and men, said Jenny Moshak, head athletic trainer for the University of Tennessee, which has one of the most dominant women's basketball programs in the country.

"Women's hips are wider so they can give birth. Because of the wider hip situation it puts more stress on the knees," said Moshak.

While nothing can be done to increase the size of a woman's knee ligament or narrow the size of her hips, sports doctors believe there is much that can be done to strengthen women's leg muscles and improve their jumping techniques.

Women's professional basketball teams are scrambling to prevent ACL injuries with conditioning and stretching exercises. "I try to simulate athletic movements" during a basketball game and "emphasize correct posture and proper alignment," said David Donatucci, a strength and conditioning coach for the WNBA's Cleveland Rockers. He also has his players do strengthening exercises for their thighs, hips, hamstrings, abdomen and lower back and to get them flexible enough to withstand twisting and turning, starting and stopping, on the court. A study at eight high schools in Cincinnati last year showed that just six weeks of a preseason training program for women coincided with a threefold decrease in serious knee injuries, compared with female athletes at the schools who did not get the training.

"This is the first time that we are starting to show a reduction in injuries. We have found that a lot of young ladies do not have the same muscular control their male counterparts do. But when you train them for six weeks, we found that we could get tremendous improvements in jumping and balance," said Frank Noyes, an orthopedic surgeon and director of the Cincinnati Sportsmedicine and Orthopedic Center.

Like many orthopedic researchers, Noyes said he doubts that training will ever be able to compensate completely for physiological differences between men and women, reducing the rate of ACL injuries among premier female athletes so it matches that of men.

Perhaps the simplest and most effective way to head off injury, said Wojtys, is to start girls playing sports at the same early age as boys.

"Look at what young boys do. They play ball starting at age 4, when they begin to learn the protective reflexes needed for safe jumping," said Wojtys. "If you want your daughter to play college basketball, you should give her a basketball at age 4. If you get her involved in sport when she is 13 or 14, that is far too late."

Staff writer Athelia Knight contributed to this report.