Keep Your Knees Healthy

Three years ago, Linda Morse tossed a tennis ball gracefully overhead, reached for it with her racket, planted her left leg on the hard tennis court for leverage, and felt something snap. "It was one of those 'Oh, no' moments," says Morse, now 73. "Most people I know who play tennis have some kind of knee trouble, and now, I just knew, I did, too."

The largest joint in your body, the knee is a complex interweaving of ligaments (which attach bone to bone), tendons (attaching bone to muscle), bones (principally the tibia, or shinbone; the femur, or thighbone; and the patella, or kneecap), and cartilage, a specialized tissue that provides cushioning. You have two types of cartilage in your knees. The first is the meniscus, a small, crescent-shaped pillow of squishy material that sits between the tibia and femur and acts as a shock absorber; there are two menisci, one on each side of your knee. The second type of knee cartilage is articular cartilage, a grand-sounding name for the thin layer of cells that covers the ends of the bones, like socks, and serves about the same purpose: keeping the bone ends safe and giving them a smooth, frictionless surface to move against.

But the knee's intricacy also makes it vulnerable. An injury, even one you suffered years ago and forgot about, can render your knee slightly unstable, like a car that's rattled itself out of alignment. The tissues holding the knee together shorten, pulling the bones out of their ideal positioning. To imagine what happens next, think of your misaligned car's balding, frayed tire tread. That's what the cartilage looks like inside an unstable knee. If it wears away completely, you have bone-on-bone arthritis, a painful condition.

Aging doesn't help. "By your 40s or 50s, the meniscus is becoming dehydrated," says Frederic Nicola, M.D., an orthopedic surgeon in Marina del Rey, California. This makes it more prone to a tear. In addition, as you get older, cartilage cells don't divide and reproduce as well as they once did, which causes the cartilage around your knees to grow thinner and less resilient. If you're not careful, the same movements you've always made — walking, serving a tennis ball, going up and down the stairs — can tear or bruise the knee's tissues.

So how can you keep your knees agile and pain-free?

Move more

"I know it sounds counterintuitive, especially if your knee is sore, but the most important thing for knee health is to be active," says Leigh Callahan, Ph.D., an associate professor with the Thurston Arthritis Research Center at the University of North Carolina. A knee that isn't used stiffens; the muscles around it start to atrophy, and because these muscles would otherwise absorb some of the shock that moves up the leg with every step, a stiff knee has to take on more of the body's weight than a supple one.