Pain when straightening the knee may be due to the following reasons: 1. It may be caused by an old injury to the knee joint. Both periarticular fractures and intra-articular injuries may result in limited joint motion and significant pain when the joint is straightened if the patient does not get enough functional exercise during the recovery process. 2. 3. It may also be caused by a peripheral nerve lesion, such as severe sciatica, where the patient takes a protective position by curling up the lower limbs. If the patient is asked to straighten the knee joint, it will induce severe pain and cause the patient to actively curl the lower limb and not straighten the knee joint to relieve the pain. 4. It may be caused by long-term braking, which is often seen in paralyzed patients. In this case, the patient keeps the knee joint in a curled and flexed position for a long time, which may cause adhesions inside the joint, resulting in the knee joint not being able to straighten and causing significant pain when straightening.