The best way to protect yourself is to strengthen the muscles that support the knee: the quadriceps, in front, and the hamstrings, in back. These two moves require nothing but a chair, says Susan Hood, a certified personal trainer in Fairfield, CT:
For a leg extension to strengthen quadriceps: Sit in a chair, with your back straight, abdomen tight, and shoulders relaxed. Rest your hands on the sides of the chair to maintain your posture. Lift your right knee until it's slightly higher than the left, then straighten your leg from the knee without moving your thigh; hold for three seconds. Slowly bend your knee without allowing your foot to touch the floor; repeat 12 to 15 times. Repeat with your left leg.
For a hamstring curl: Stand with your feet shoulder-width apart, knees relaxed and slightly bent and abs tight. Rest your hands on the back of a chair placed in front of you for balance. Flex your right foot and bring it toward your buttocks, squeezing the back of your leg on the lift. Hold for three seconds. Slowly lower foot but don't touch the floor; repeat 12 to 15 times. Switch legs. Do two sets of 12 to 15 reps for each move, every other day, for six weeks. Work up to three sets; if it's easy, add three- to five-pound ankle weights.