The knee is a relatively simple joint required to do a complicated job…to provide flexible mobility while bearing considerable weight. While walking down the street, our knees bear three to five times our body weight. When climbing upstairs, that force can multiply to seven times our body weight. That force is borne by compact structures of bone and cartilage, supported by muscles and ligaments. When the knee is overstressed in sports or in everyday activities, these structures can break down — and knee injury occurs.