What does it mean to hyper-extended your knee?

Injured by extension: Hyperextension means over-extension, or straightening a knee joint beyond the straight position. If one straightens out his leg, it stops approximately straight, which is normal. If his lower leg and foot continues moving past the straight point, that is called hyperextension. At first, knee ligaments will over-stretch, with a little bleeding. Further extension will tear up the ligaments.

Bend beyond straight: A fully extended knee usually gets to the straight position or perhaps a few degrees past straight. A hyperextended knee occurs when the knee goes beyond the patient's normal amount of full extension. Hyperextension of the knee can result in significant pain and injury. Evaluation by your physician is usually recommended.