Knee popping may be due to meniscus injury, but other causes such as physiologic causes and other diseases (e.g. chondromalacia patella) cannot be ruled out. 1. Meniscus injury: Due to a tear in the meniscus, the free portion of the meniscus may get stuck between the femur and tibia, resulting in symptoms of meniscus popping. 2. Physiological causes: mainly seen in the case of bursting of fluid bubbles in the joint cavity. 3. Other diseases: such as chondromalacia patella, free body in the knee joint, osteoarthritis of the knee joint, cruciate ligament old injury, etc., due to the cartilage surface is thin and rough, the joint activities will make a sound when rubbing against each other. In addition to the above reasons, running knee can also cause knee popping, mainly due to running patella deviation from the normal position of other parts of the impact caused by popping. It is recommended to go to the hospital to find out the cause of the problem and take regular treatment under the guidance of the doctor.