When this symptom occurs, the first thing to consider is damage and lesion of the patellar ligament. The patellar ligament is a thicker ligament on the front side of the knee located below the patella, or kneecap. When the knee is in semi-extension or semi-flexion, the patellar ligament is more tense and obvious, and the patellar ligament mainly controls the movement of the knee joint in extension. If there is some chronic damage to the patellar ligament, the local blood circulation will be poorer, and when the knee joint is not moving for a long time and the knee is suddenly straightened, the patellar ligament will contract and stimulate the local area, causing significant local pain. If there is pain inside the knee joint, it may be caused by the stimulation of the synovial membrane of the knee joint. The knee joint bending state is the most unstable state of the knee joint, which will cause the blood circulation of the synovial membrane to become impaired, and the sudden straightening will stimulate the local pain.