Knee hyperextension is thought to cause as much as a fivefold increase in the risk of injuring the ACL.
A common cause is a straight leg receiving a severe blow that forces the knee backwards, for example during a car crash. This usually results in injury to several knee ligaments and possibly dislocation of the knee.
Hyperextension can also happen as a result of a fall, or while playing a sport that puts great stress on the knee. Examples of such sports are volleyball, football, basketball or gymnastics. This happens less frequently and usually only the anterior cruciate ligament (ACL) within the knee is damaged.
If the quadriceps muscles at the front of the thigh are weak, the hamstring muscles at the back of the thigh will compensate, leading to an imbalance between the two and an increased likelihood of the joint being pulled out of position backwards - hyperextension.
Symptoms:
Depending on the severity of the injury, your symptoms may include:
Pain felt at the back and sides of the knee
Possible ‘pop’ sound or feeling at the moment of injury
Swelling that begins within three hours of injury
Instability of the knee and a feeling that it might give way
Decreased range of motion of the knee