Knock knees, locked knees, knobby knees, trick knees. Hiker's knee, runner's knee, surfer's knee, dancer's knee. They all take a beating.

The knee is the body's largest joint but far from the strongest, especially considering all the biomechanical demands on it.

"It's just two tables of bone held together by ligaments on the sides and muscles fore and aft," says Dr. George Sheehan, a cardiologist and author of several best-selling books on running.

"It's the most exposed joint of the lower extremity," says Dr. Kim Sloan, an orthopedic surgeon, director of sports medicine at the New Jersey Medical School and consultant to the New York Nets basketball team.

Unlike the elbow, the knee must bear the body's weight. Unlike the hip, it's not protected by thick thigh muscles and pelvic bone and lacks the stability of a ball-and-socket joint. Unlike the ankle, it's not close to the ground with stabilizing support from the foot.

Though it is often described as a musculoskeletal hinge, the knee actually is much more complicated than that.

"People assume it's a hinge, and they think it just opens and closes like a door," Sloan says. "That's a misconception. It also needs to rotate."

In addition to its extend-and-flex hinge motion, the knee moves side to side and back and forth. It swivels, twists, slides and glides. With mere hinge-like knees, Mikhail Baryshnikov could not leap high off the dance floor in a grand jete' and land softly and gracefully; Tony Dorsett could not shift and cut on a dime to avoid a linebacker; Martina Navratilova could not rush the net after a serve, then pivot and race back to retrieve a baseline lob.

And for the rest of us, jogging around the block or even zeroing in on the refrigerator in a crowded kitchen would be a lot more difficult.

"The knee is a very intricate joint," says Dr. J. Richard Wells, an orthopedic surgeon and co-director of the Sports Medicine Clinic at Georgetown University Medical Center. "Derangement of any one of its parts can cause all the other parts to go bad."

Trouble is, all this complex mobility makes the knee vulnerable to stress and injury -- especially in athletes competing in violent contact sports such as football and hockey.

Knee problems are a major reason why the average career in the National Football League lasts only 4.6 years. In the 1960s, about 70 percent of professional football players had knee surgery before age 26. Many great athletes -- including football stars Joe Namath, Gale Sayers and Dick Butkus, and hockey star Bobby Orr -- had to retire early after repeated knee operations failed to repair the damage from years of punishment.

"If you stop the lower leg and the upper leg keeps moving -- whether two people kick a soccer ball at the same time or you trip on the turf or a halfback makes a cut -- you run the risk of putting more stress on the knee ligaments than they can tolerate," says Dr. John Aseff, assistant medical director of the National Rehabilitation Hospital.

Knee sprains were by far the most common injury in skiers treated at a clinic in Sugarbush North, Vt., between 1972 and 1982, according to a recent study at the University of Vermont College of Medicine at Burlington.

The knee accounts for 25 percent of all serious athletic injuries -- those sidelining the victim for at least a week -- and more than half of the serious leg injuries, says Georgetown's Wells.

But football players, hockey players and skiers aren't the only ones susceptible to knee injuries.

"I probably have 10 patients a week who come in with knee or foot problems from aerobics," Wells says.

"Often, they aren't in good shape to begin with, or they overdo it -- or both," he says. "They're into the 'Hey, I'm gonna get back in shape today' mentality."

Some knee injuries occur in middle-aged people who suddenly try to recapture the athletic glory of their youth.

They may forget the old injury that sidelined them in high school, the extra 20 pounds they now lug around and the touch of arthritis that stiffens their joints.

"And they probably never heard of stretching," adds Wells. "They may lack proper running shoes. They've got the will, the desire, the time, but they're not educated to the sport itself."

"Seventy percent of runners have some kind of knee pain," says cardiologist-author Sheehan, who at age 66 runs an average of about 20 miles a week and has had "every injury in the book."

The knee connects the two long bones of the leg -- the femur, or thigh bone, and the tibia, or shin bone. Between the two bones, acting as shock absorbers, are two crescents of cartilage called menisci. Their smooth, slippery surface -- what would be called gristle in meat -- keeps the tibia and femur from rubbing against each other and cushions the blows the knee suffers even in everyday use. Frayed or torn cartilage is the most common injury to the knee.

What protection the knee gets comes from three sets of tissue. The patella, or kneecap, covers the joint in the front. Four bands of ligaments -- front, back, inside and outside -- help stabilize the knee. And it is supported by 13 muscles, including the quadriceps group in the front of the thigh, the hamstrings in the back of the thigh and the gastrocnemius in the calf.

Still, the knee is relatively vulnerable.

"It's exposed," says Dr. Charles Epps, chief of orthopedic surgery at Howard University. "Inherently, it's not as stable as other joints. The hip is a ball-and-socket. But in the knee, you've got those two huge bones simply gliding back and forth against each other."

Many knee problems are caused or worsened by the weekend warrior's tendency to overdo exercise and ignore the warning signals of pain.

"You gotta treat them with respect," says orthopedist Sloan. "When they hurt, they're trying to tell you to slow down.

"There's a reason for pain. If it's short-lived and mild, there's probably no reason to worry. But if pain is persistent, it's trying to tell you something. You may be trying to do too much. Or you may have an injury that needs medical attention."

"When I was in the Navy in the 1940s," says Sheehan, "there were two diagnoses for knees. One was 'surgical knee.' The other was, 'Go to duty.' "

Diagnosis and treatment of knee problems have come a long way since then, and much of the progress has resulted from a new technique called arthroscopy that allows surgeons to see inside the knee without cutting it open.

The arthroscope, a narrow, lighted tube with magnifying lenses, is inserted through a quarter-inch incision, sometimes without general anesthesia. It is connected to a miniature video camera, which projects on a television-like monitor a detailed view of the inside of the patient's knee.

The surgeon manipulates the scope while watching the interior of the knee on the monitor.

Originally developed more than 50 years ago by a Japanese surgeon, arthroscopy was rarely used until the 1960s, when advances in fiber optics and miniaturization of television cameras made it more practical. Since then, it has revolutionized diagnosis and treatment of some types of knee problems.

Arthroscopy not only eliminates the need for a long, deep incision to open up the knee and reduces the risk of infection, but also gives surgeons a better picture of the inside of the knee. Insertion of the scope on each side of the knee -- and directly into tissue that otherwise would have to be cut open -- results in a nearly complete image of the joint.

"Through that little tiny incision," Wells says, "I can see everything inside the knee."

The arthroscopy patient may be spared painful, tissue-cutting surgery, hospitalization and a longer convalescence. Some patients who undergo arthroscopy walk out of the hospital within a few hours.

The arthroscope still is used mainly for diagnosis, but in some cases it also can treat a knee problem. For example, fragments of bone and bits of torn ligaments or cartilage can be removed by arthroscopy. Rough surfaces of bone -- for example, where arthritis has worn away part of the inside of the kneecap -- can be shaved and smoothed to reduce irritation. And some cartilage and ligament tears can be repaired through arthroscopy.

One of arthroscopy's greatest success stories is Joan Benoit who, a year ago last spring, was on a routine 20-mile run, training for the Olympic trials, when she felt a twinge in her right knee. A mile later, she had to slow to a walk. A shot of cortisone and a day off allowed her to resume training, but 10 days later she was sidelined again with inflammation of the knee.

Seventeen days before the Olympic trials, Benoit underwent arthroscopic surgery to remove the inflamed tissue from her knee. She resumed running within a few days, won the Olympic trials and then went on to win the gold medal in the first women's marathon at the Olympic Games in Los Angeles.

But such highly publicized cases as Benoit's pose a problem for doctors. Patients reading about such near-miraculous recoveries tend to assume that their own knee problems can be treated easily and repaired overnight. For many, that's not so.

"The biggest problem right now is people thinking you can fix anything with an arthroscope," says Dr. Robert Kerlan, an Inglewood, Calif., orthopedic surgeon who works with the Los Angeles Rams football team and the Los Angeles Lakers basketball team. "Nothing could be further from the truth."

"I run into that problem all the time," says Sloan. He tries to explain to patients that every case is different, that their muscles may not be as well developed as a superbly conditioned professional athlete, and that they may not have the time or the incentive to dedicate their lives to the often-tedious job of rehabilitation.

"I've had athletes go back onto the playing field five or six days after arthroscopy for torn cartilage," says Sloan, "but it's not what I recommend."

Arthroscopy has become one of the most commonly performed surgeries, even though less than 5 percent of all serious knee injuries require surgery. More than 200,000 arthroscopic operations are done every year.

Some doctors worry that arthroscopy is overused, or used by surgeons who lack sufficient training in the use of its highly sophisticated technology.

"When you have a new hammer, sometimes everything looks like a nail," says Dr. Robert Nirschl, medical director of the Virginia Sports Medicine and Rehabilitation Center in Arlington.

"It's one of the most common operations -- and one of the most abused," says Wells. "People should remember that it's not a totally benign operation. It has risks and some pain. It requires spinal or general anesthesia, and too many doctors doing it lack proper training."

With knee injuries, as with other kinds, successful treatment depends not only on first aid or surgery but on diligent rehabilitation. "Rehab" takes time, and it may mean a lifelong change in habits -- for example, losing weight, performing regular exercises or giving up rugby for tennis, or tennis for walking.

Sometimes, rehab begins even before surgery. Doctors may put a patient through six or eight weeks of rehabilitation to strengthen the knee before elective surgery to correct an injury. And rehabilitation sometimes enables an otherwise healthy patient to avoid surgery altogether.

The same injury affects different individuals differently, depending on their condition, lifestyle and occupation.

"You don't treat a weekend warrior or a person who falls on the ice in the parking lot the same as you would a world-class athlete," says Aseff, of the National Rehabilitation Hospital.

Wells uses the example of a skiing accident that tears the anterior (front) ligament of the knee.

"If it's a weekend warrior, maybe we can rehabilitate him," says Wells. "If it's Gale Sayers, it might end his career."