Hyperextension of the knee can lead to painful injuries to the ligaments and muscles. When the knee extends beyond the point of being fully extended, past it's normal 'straight' resting position, the posterior cruciate ligament can be damaged, and other ligaments too. This sort of injury usually occurs in high impact sports, or sports that require lots of movement. It may even happen in day to day life if you happen to fall or have a misstep.