She was charging for the ball in the midst of a heated basketball game last summer. Suddenly, another girl's knee smashed into hers.

``I fell to the floor,'' recalls O'Connor. Right away, ``it got all swollen. I was crying.''

Her parents took her home and put ice on her knee, and at first that seemed to do the trick. But a week later, she tried to play again. Her knee really hurt, she recalls. Perhaps even more ominous, it kept buckling as she played.

Her parents took her to one doctor, then another. The diagnosis? A torn ACL, or anterior cruciate ligament, the most common severe joint injury in sports today, and a particular plague for vigorous young women like O'Connor.

A generation ago, it was rare to see a healthy young woman limping in to see her doctor with a knee wrecked in friendly combat on high school or college playing fields.

Today, a knee freshly bandaged from surgery or a high-tech knee brace has become a sign of the times for young female jocks, the kind of dubious honor that shows both how far women have come in crashing the barriers to full participation in sports -- and the price some are paying for their shot at athletic glory.

In 1982-83, for instance, there were 80,000 female athletes competing in college sports, according to the National Collegiate Athletic Association. By 1994-1995, the figure had swelled to 110,500, with a 9 percent increase in 1989 to 1992 alone, the NCAA says.

In high school sports, the figures are even more dramatic. In 1971, before the federal law known as Title IX mandated that girls and boys get comparable athletic opportunities, fewer than 300,000 girls participated, according to the National Federation of State High School Associations.

A year later, thanks to Title IX, the number had soard to more than 800,000. Today, there are 2.4 million.

The downside of all this forward motion, sad to say, is that men and women now have a chance to injure themselves at roughly similar rates in many sports.

But certain sports, especially those like soccer and basketball that demand a lot of pivoting and fast changes of speed and direction, seem to take a higher toll on women than on men.

Especially on the anterior cruciate ligament in the knee.

In one recent study, Dr. Elizabeth Arendt, medical director of men's and women's varsity athletics at the University of Minnesota, and Randall Dick, associate director of sports sciences at the NCAA, documented striking gender differences in ACL injuries in soccer and basketball, sports in which the rules are comparable for men and women.

Overall, they found, the rate of the injury during the five-year study was three to four times higher among females than males. In basketball, women were three times more likely to rip their ACLs; in soccer, twice as likely. Most of the damage was from non-contact, twisting injuries.

``It's peculiar why this shows a gender or sex difference,'' says Arendt. In fact, ``It's rather alarming that we don't know the answer, but we don't know it for men, either.''

Part of the reason the anterior cruciate ligament takes such a beating is that evolution hasn't yet come up with a better solution for a joint that must be both flexible and capable of carrying tremendous weight.

This means that you may be asking for knee trouble if you play hard, both in contact sports like football and some non-contact sports like skiing. In skiing, modern technology -- in the form of boots that lock the leg in a forward position -- actually makes things worse by making the ligament vulnerable to rips if you fall over backwards.

But for growing numbers of young female basketball and soccer players -- and for many researchers -- the most pressing question is why the risk of the ligament injury is especially acute for women.

One theory is that women's wider hips cause the femur, or thigh bone, to come into the knee at a broader angle. Men's hips, knees and feet usually line up straight, says Nicholas Giurleo, director of physical therapy at Concord Avenue Physical Therapy Associates in Cambridge.

But in women, he says, wider hips may make the knees slightly ``knocked'' or tilted inward, which can make the joints more vulnerable.

Wider hips may also explain why women's kneecaps slide around too much side to side, adds Lori Thein Brody, a physical therapist at the University of Wisconsin in Madison.

Another theory is that women and men may have a different balance between the quadriceps muscle on the front of the thigh and the hamstrings at the back.

In general, men have bulkier muscles because they have higher levels of male hormones, but even in women with impressive quads, the hamstrings may be relatively weak. Because the hamstrings work with the ligament to stabilize the knee, if they are weak or underused, the knee may be in jeopardy.

Women are also more loose-jointed than men because female hormones make some ligaments stretch, a decided benefit during childbirth but a risk if knees are too loose.

Regardless of why women's knees are more vulnerable, once the ligament is damaged, the best treatment is to let the knee heal for four to six weeks so that the joint regains its range of motion, then to operate, says Dr. Bertram Zarins, chief of sports medicine at Massachusetts General Hospital.

In surgery to reconstruct torn anterior cruciate ligaments, which is performed more than 10,000 times a year and is considered 90 to 95 percent successful -- the repairs are usually made through small viewing tubes called arthroscopes inserted into the knee.

In addition, doctors often make a two-to-three-inch incision to ``harvest'' a small piece of the kneecap tendon to replace the damaged ligament.

In the procedure, doctors snip off the dangling ends of the damaged ligament, then take a third of the tendon that anchors the kneecap -- with small pieces of bone still attached at either end -- and screw it into tunnels made in the ends of the thigh bone and the tibia, where the ligament was anchored.

The graft ``is almost the same size as the ACL and is the same strength,'' says Zarins, adding that the tendon from which it was taken regrows and regains its original strength. Though most reconstructive surgery uses this technique, some Boston surgeons use a piece of the hamstring instead of the patellar tendon.

Like Meaghan O'Connor, some patients who have this surgery spend 23 hours a day for a week with the knee in a continuous passive motion machine to keep it from freezing up as it heals.

The encouraging news, say orthopedic specialists, is that while recovery from ACL surgery used to take a year, accelerated rehabilitation programs now allow athletes to return to sports in about six months.

Even that, of course, is a long time in the life of a healthy young athlete like O'Connor, who has been chafing at the bit since her surgery in August.

She plans to run track this spring to get in shape for basketball next summer. For her, as for many young female athletes, the joy of sports is well worth the risk.

In fact, when her doctor told her that the only way to make sure she didn't re-injure her knee was to quit basketball, she had her answer all ready: ``I didn't want to do that.''

SIDEBAR 1:

PLEASE SEE MICROFILM FOR CHART DATA

GLOBE STAFF GRAPHIC/DAVID BUTLER

SIDEBAR 2:

Some tips to avoid injury

While many of these tips can help athletes avoid knee injuries in general, they are designed for female athletes who want to avoid injury to the ACL, or anterior cruciate ligament.

1. Coaching. Get a good coach as soon as you start doing any sport seriously. Parents don't count, unless they're trained as coaches. Ask your coach, nicely, what credentials and training she or he has had.
2. Skill level. Play with people at your level. In basketball, if you play with klutzes below your level, they may make up for their lack of skills with brute force. And if they can't move as fast as you, you may end up tripping over them. On the other hand, if you're the klutz, you may twist yourself into knee-wrecking contortions to keep up.
3. Fatigue and pain. Don't play when you're exhausted and don't play through real pain. Among other things, this means distinguishing between ``I'm sick of this and I want to go home'' feelings and the sharp, stabbing pain of genuine injury, a pain that comes back as soon as you start playing again.
4. Shoes. It's common sense to wear shoes with good arch support and shock-absorbing capacity -- in sports as well as the rest of your life. But studies are mixed on whether shoes can be blamed for women's ACL injuries. If your shoes are too ``good,'' that is, give you too much traction, your foot won't give way on fast stops, and the resulting torque on your knee can rip the ligament.
5. Posture. For many women, the ``default posture'' -- how you stand when you're not thinking about it -- is with the knees hyperextended or arched slightly backward. Some researchers think that if you habitually stand this way, you may land wrong on jumps, too.
6. Landings. Many ACL injuries could be prevented if athletes were taught to land correctly from jumps -- with knees bent, not hyperextended, and with feet under the hips. Landing on bent knees gives the hamstrings a chance to stabilize the knee.
7. Fast stops, cuts and turns. When you have to decelerate and change direction fast, take three small ``stutter steps'' instead of one big one. When this three-step stop was taught to two major women's basketball teams, the incidence of ACL injuries dropped 89 percent over several years.
8. Weights. Get to the weight room. Take the time -- three sessions a week -- to build up your leg muscles, especially your hamstrings. Try to work with a trainer certified by a recognized organization such as the National Strength and Conditioning Association. If you think you're headed for trouble, get a physical therapist to check you for any abnormalities in your gait and imbalances between muscle groups.
9. Warm-ups. Never start jumping and pivoting hard unless you're thoroughly warmed up, i.e., actually sweating.
10. Stick with the sport you love -- sensibly. It may make sense to switch sports if you think the one you've chosen is too risky. But don't let the naysayers get you down too easily.

Decades ago, when women first started competing in long distance running events, half fainted at the finish, prompting the International Olympic Committee to conclude that women were too delicate for the tough stuff, says Dr. Elizabeth Arendt, medical director of men's and women's varsity athletics at the University of Minnesota.

That's a ``Neanderthal'' attitude, she says. Life is inherently risky, and "people play a sport because they love it."