Your knee is a mechanical wonder, enabling this largest of weight-bearing joints to roll, glide, and rotate. Unfortunately, its also the most used and abused of joints, too often injured by sudden trauma or long-term wear and tear. To help protect your knees from premature breakdown, you need to keep the supporting muscles and ligaments strong and flexible. Here are four exercises that will help. Some may not be appropriate if you have existing knee problems. And if you feel pain or stiffness when doing any of these exercises, go easy. If the pain gets worse, stop until youve talked to a doctor or therapist.