Knee popping with leg extension may be associated with physiologic causes, pathologic factors such as lateral patellar compression syndrome, osteoarthritis and other causes. 1. Physiological reasons: the pressure of gases and liquids inside the normal joints tends to change with the degree of joint mobility. Changes in the pressure inside the joint cavity can lead to the rupture of gas bubbles, triggering different degrees of popping. 2. Lateral patellar compression syndrome: Lateral patellar compression syndrome is a pathologic factor that can easily lead to stretching of the knee. Due to the patella in the development stage, easy to favor the femoral epicondyle part, and with the femoral epicondyle wear and extrusion, in the extension of the leg will produce knee ringing. 3. Osteoarthritis: Osteoarthritis of the knee joint will reduce the synovial fluid between the joints, the joint surface is not smooth, resulting in abnormal friction of the joint surface, which will lead to the phenomenon of stretching the leg and knee ringing. If the ringing in the knee occurs repeatedly and is accompanied by pain and discomfort in the knee joint, it is recommended to consult a doctor for a clear diagnosis and necessary treatment.