Casualties always remember the moment their knee gave out. Charles Barkley, the former All-Star member of the Houston Rockets, knew immediately that his long career in the National Basketball Association had ended after tearing his left quadriceps tendon during a game in Philadelphia last December. Barkley told an Associated Press reporter: "I knew it was over when it first happened. I saw the way the kneecap was bulging through my leg and I said, 'Well, it's been fun.' "

Barkley isn't the only elite athlete to hit the ground after a serious knee injury. NFL running backs Jamal Anderson, of the Atlanta Falcons, and Terrell Davis, of the Denver Broncos, both suffered knee injuries last fall. Both players tore the anterior cruciate and medial collateral ligaments in their knee. It's unclear how effective these players will be if and when they return to football.

Barkley, Anderson, and Davis are among dozens of high-profile athletes in recent years who have heard an ominous popping sound coming from their knee. This noise heralds serious injury and occurs with alarming regularity to athletes of all abilities, from weekend warriors to highly skilled professionals. In fact, knee injuries account for a quarter of all sports-related injuries.

Dozens of studies of the knee joint have been done in recent years, and while many of these studies document the problematic nature of the knee, far less information is available on what you can do to keep your knees healthy. Keeping your knees in good repair actually can be worked at. But like anything worth doing, it requires commitment and a bit of common sense.

Built to blow

Before you learn how to preserve your knees, it's good to understand why the knee joint fails so often. Simply put, the knee wasn't designed to handle the intense pressures that occur in modern sports, like professional basketball. But you don't have to be Michael Jordan to put tremendous strain on your knees. In fact, in activities such as stair climbing and running, the pressure on the knee can be four to six times body weight. That means when a 180-pound man goes for a quick jog in the park, the force slamming through his knee is actually close to 900 pounds, equal to the weight of more than three Hulk Hogans.

In a high-speed, quick-turning game like professional basketball, the forces become drastically higher. It's not surprising, then, that burly muscle men like Barkley end up on the floor writhing in pain. The knees are vulnerable because they are unprepared for the stresses that modern athletes of all statures put on them. Imagine two turkey drumsticks separated by a layer of hard jello, connected by a few stretchy strands of pasta, and you've got a pretty decent model of the knee.

The lighter the better

Keeping those drumsticks in shape requires a healthy attitude toward turkey, dressing, pumpkin pie, and lots of other foods. Indeed, one of America's foremost authorities on the knee joint blames many knee injuries on a common piece of household furniture. According to James Fox, M.D., "The biggest cause of knee injuries in America is the dining room table" -- or at least, the activity that occurs there.

"People are too heavy," says Dr. Fox, a board-certified orthopedic surgeon at the Southern California Orthopedic Institute and the author of Save Your Knees, a critically acclaimed book published in 1988 and recently reissued. Carrying extra weight increases the wear and pressure on the knees. Since normal forces on the knees are already high -- remember those three heavy wrestlers pounding through the knee during every jog -- it makes sense, he says, to reduce the amount of weight you are putting on them.

In addition to a healthy diet, Dr. Fox, who has been operating on and rehabilitating knee patients for decades, offers another low-cost prescription for prevention of injury: exercise. Exercise strengthens the muscles in the thigh, which in turn help stabilize the knee joint. To illustrate his point, Dr. Fox likens the knee to a radio antennae. If an antennae were "just fixated to the ground, it would be unstable. The muscles around the knee are like the guy wires holding up the antennae; they diminish the pressure at the base."

To keep your knees strong and stable, Dr. Fox recommends using exercise machines like the leg press and the stair climber. He likes them because they strengthen the leg muscles without putting much strain on the knee. He also prescribes non-impact sports such as cycling, swimming, and cross-country skiing.

Asked for a one-sentence summary of his best advice, Dr. Fox responded, "Stay in shape." It's an old prescription but a good one. And it just might help your knees survive.

Related Links:

The University of Washington's Department of Orthopedics maintains a comprehensive website dedicated to joint health, and contains a good series of simple knee exercises.

The American Running Association website offers a wide range of weight training and stretching exercises.