Nearly half the adults in America will develop osteoarthritis by the time they're 85, and obesity will be the main culprit. You can protect your knees by staying active and strengthening the right muscles—at any age—but take these precautions:
Lighten the load. Simply walking around puts pressure equal to three to five times your body weight onto your knees, and toting around extra pounds makes the burden even worse. Excess weight on your knee joints may accelerate osteoarthritis, the degeneration of the joint, so shed some of those pounds!
Work all the muscles. Strengthening and flexibility exercises can build up muscles to stabilize the knees—but don't bulk up one group and forget about the others. Most important are the quads (front of thigh), hamstrings (tendons and muscles behind the knee and thigh), and hip abductors and adductors (outer and inner thighs, respectively).
Pick your sports. Rowing, cross-country skiing, and cycling are knee-friendly activities because they are low impact and don't encourage twisting.
Put it on ice. Don't ignore a tweak or strain, no matter what sport you take up. This is a simple way to reduce inflammation and pain.
Avoid rotation of hips and knees. Jumping and slowing down from a run—common to many exercises—seem to be particularly bad for the knee's ACL (anterior cruciate ligament). Land with joints aligned so that hips are over knees, knees are over ankles, and ankles are over toes—but with some give to the joints. Young female athletes who play sports like soccer and basketball tend to injure their ACL five to eight times as often as young male athletes.
Beware the up and down. Running in hilly terrain can put particular strain on tendons in the knees. Pay attention to clues that your knees need a rest—or at least a flatter course.
Cross-train. Overuse is the slow and steady way to knee injuries. Opt for a range of varied activities by alternating workouts. If you run, for example, alternate with cycling to use different muscle groups.
Brace yourself. Braces typically don't prevent injury, but if you're rehabbing an old injury or are trying to prevent repeated tears or strains, having your doctor fit you for the right brace may be in order.