Rattling sound in knee extension is considered to be related to physiological reasons, osteoarthritis, contracture of iliotibial bundle, etc. The details are as follows: 1. Physiological reasons: when the knee joint is extended, the joint cavity is enlarged, and the gas dissolved in the synovial fluid precipitates out and forms bubbles, which rupture or vibrate with the synovial fluid, and then a crisp ringing sound will be made, which is a physiological phenomenon. 2. Osteoarthritis: Osteoarthritis patients, there will be wear and tear of joint cartilage. At the same time, due to pain and lack of activity, there will be muscle weakness, which will cause meniscus damage, and will also cause the knee extension and flexion sound. 3. Iliotibial bundle contracture: The popping sound when the knee is flexing and bending is caused by the instantaneous change of the tension of the iliotibial bundle, which leads to the tibiofemoral joint impingement. 4. Other causes: for example, meniscus injury, ligament laxity, etc., can also cause knee extension and flexion sound. Physiologic causes do not cause joint pain or discomfort and do not require treatment. If it is a disease that causes the knee to rattle, you need to go to the hospital to further identify the cause of the disease, and then targeted treatment.