By Mary Brophy Marcus

It happened in an instant. Last year, Deb Anderson was chasing a soccer ball down a field one moment, and the next moment her knee gave out and she was on the ground. "It was the oddest feeling. My foot went one way, and the rest of my body went the other way," says Anderson, 44, an editor and writer at the Oakland Press in Pontiac, Mich., who played soccer four nights a week. Anderson had made the unhappy discovery that the knee is women athletes' Achilles heel.

As more women take up sports-and as early waves of fitness-minded women age-female knee injuries have surged, reaching almost 1.4 million a year by the late 1990s, double the number a decade earlier. That's still 100,000 fewer knee injuries than men rack up each year, because fewer women play sports or work out. Yet those who do exercise run a much higher knee risk than their male counterparts. Women's rate of anterior cruciate ligament (ACL) tears-Anderson's diagnosis-is four to six times that of men who play the same sports. But as researchers study the biomechanical differences that make women more prone to knee damage, they're coming up with exercise programs that can reduce the risk and help those who have suffered damage get back in the game.

Busted-knee boom. The knee is at best a vulnerable joint: two bones butting head to head, lashed together with slim, ropelike ligaments and cushioned with pads of cartilage called meniscuses. Cartilage lubricates the back of the kneecap, and tendons-which connect muscle to bone-run in front and back of the knee. People who play basketball, volleyball, and soccer-any sport where there's lots of pivoting and jumping-tend to injure ligaments or rupture a meniscus. Runners injure the kneecap and inflame tendons. In each case, female athletes-fitness nuts and weekend warriors alike-suffer more than males.

Doctors are just beginning to understand why, says Jo Hannafin, orthopedic director of the Women's Sports Medicine Center at the Hospital for Special Surgery in New York City. Some researchers blame hormone cycles, which might cause the ligaments to soften at the monthly estrogen peak. But Hannafin is skeptical. "Intuitively, it doesn't make sense to me that a ligament could get weaker or stronger in a two-day period."

Focus is shifting to anatomical factors. In women, the thigh bone angles inward from the hip to the knee-the so-called Q angle-more sharply than in men. That makes women slightly knock-kneed, which stresses the joint. Research has shown that a larger Q angle makes an athlete more prone to knee injury.

While there's nothing a woman can do about her Q angle, many studies point to another factor that women can control: muscle strength. Strong, well-coordinated muscles can stabilize a knee, especially in jumping, twisting sports such as basketball and tennis, says Frank Noyes, director of the Cincinnati Sportsmedicine Research and Education Foundation. Women's legs tend to be weaker than men's, and they often suffer from a strength imbalance: The quadriceps, the massive muscles at the front of the thigh, tend to be much stronger than the hamstrings, at the back-an imbalance that may also reduce knee stability.

Beating the injury odds. By building up the hamstrings and the inner quadriceps, the muscles inside the thighs, and relearning landing and twisting techniques, women can beat their biomechanical drawbacks. In a study last year Noyes and several colleagues enrolled women planning to play soccer, basketball, or volleyball in a six-week program prior to competition season. The program, which included stretching, jumping, and weight lifting, cut their knee-injury rate by more than two thirds.

The Cincinnati scientists have now opened the program to the public, but you don't need to go to Ohio to lower your knee risk. A few sessions with a reputable certified trainer, at about $40 to $60 a pop depending on where you live, can help serious and casual athletes plan a knee-protecting regimen of stretching and strengthening for the calf, hamstrings, and quadriceps.

For women who do serious damage to their knees-a ruptured ligament or a torn meniscus-there may be no alternative to an operation. But surgery need not mean the end of weekend bike trips and pickup soccer games. Doctors can replace a section of ruined meniscus or patch a tear with a meniscus graft, taken from a cadaver. And ACL surgery, which 20 years ago required a week's hospitalization and a year or more of rehabilitation, is now an outpatient procedure; rehabilitation times have been cut in half.

Anderson had ACL reconstruction a year ago May and was back on the soccer field after five months of intensive rehab. "At first, I was such a wreck, worrying about reinjuring my knee. I play somewhat cautiously still. But truthfully, I just keep looking at those ladies in their 50s playing out there with me and think, 'I want to be like them,' " she says.

Listen. Don't ignore crunching and popping sounds; see a doctor or physical therapist.

Don't get dependent. Knee braces can offer protection but can cause muscle atrophy if overused.

Train. Visit this story at www.us news.com for links to knee exercises and other online information.

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