Each day, the knees take a pounding. Walking, running, jumping, kneeling, pivoting, leaning. The knees must bear the body's weight.

Ironically, the knee -- the body's largest joint -- is also the most vulnerable and unstable, according to James M. Fox, an orthopedic surgeon in Van Nuys, Calif. It is not designed to withstand the stresses and strains of modern life.

"We need a joint that's rugged, like the hip, which is a ball and socket design with inherent stability," writes Fox in his 1988 book "Save Your Knees" co-authored with Rick McGuire.

"Instead, we get two giant bones propped on top of one another, held together by the anatomical equivalent of rubber bands."

Although most people believe the knee operates like a hinge on a door -- swinging open and closed -- it is really more complicated than that.

The knee connects the thigh bone -- the femur -- with the leg bone -- the tibia. The two bones are held together by four major ligaments -- bands of fibrous, slightly elastic tissue.

The ends of the thigh and leg bones are wrapped in cartilage -- gristle-like material that acts as a natural shock absorber and prevents the two bones from rubbing against each other.

In the front of the knee is the kneecap -- or patella -- which helps protect the area from harm.

Not only does this structure allow the knee to swing back and forth in a rocking motion, the joint can also roll, twist and slide.

Any number of things can go wrong with the knee. Bones break, ligaments snap, cartilage tears and the kneecap can slip out of alignment.

Knee injuries are not uncommon. Fox estimates that about 50 million Americans have experienced a knee injury or knee pain.

The weekend exercise warrior who decides to become fit in twenty minutes is particularly vulnerable to knee injury, said Carl L. Stanitski, an orthopedic surgeon at the University of Pittsburgh. He urges people to regard fitness as a goal, not an event.

"It is not realistic for a person accustomed to walking only as far as the refrigerator to suddenly go out and walk 10 miles," he said.

To maintain healthy knees, Fox recommends that people:

Shed pounds if overweight.

Condition the leg muscles by exercising regularly.

Avoid excessive squatting or stair climbing.

Limit walking in high heels or thin-soled shoes.

Change positions frequently when sitting for a long time.

If the knee becomes very painful and swollen, Fox suggests seeing a doctor promptly.