According to the Western Journal of Medicine, the knee is stabilized by the shape of the joint and the ligaments accompanying it. The joint is shaped by the local bone protuberances and the surrounding cartilage. The many ligaments that support the knee joint include the cruciate (cross-shaped) and collateral ligaments.
The Western Journal of Medicine article goes on to say that if a ligament is injured, the knee joint can become unstable. This causes the knee to flex or extend beyond its normal range and causes bruising and swelling. Knee ligament injury is common in many athletes and can be difficult to heal. Strengthening the muscles in the legs can decrease the risk of ligament injury.