The sound of bones rattling when the leg is straightened is usually referred to as knee popping sound. The causes of knee popping sound can be divided into physiological causes and pathological causes, pathological causes. Knee popping sound refers to the knee joint activities, the surrounding tendon sliding or joint impact and sound, this situation often suggests that some parts of the composition of the joint changes. 1. Physiological causes: Normal adult knee joints often make a crisp popping sound if they are suddenly straightened after a certain period of time at rest. Physiologic popping does not require treatment in the absence of any pain or discomfort. 2. Pathological causes: it refers to a series of pathological changes in the knee joint due to joint injury, disease or structural variation, in which case pathological popping can occur when the diseased tissues rub against each other during exercise, and most of them are accompanied by pain or discomfort. It is commonly seen in knee meniscus injury, osteoarthritis, degeneration of articular cartilage and other diseases. When knee popping occurs when the leg is straightened and is accompanied by pain or other discomfort, it is recommended to consult a doctor in time to clarify the cause of the problem and receive appropriate treatment.