The pain behind the knee joint when you get up after squatting for a long time is due to a slight strain and strain on the biceps femoris, semitendinosus, and semimembranosus muscles. The main reason for this is that after squatting for a long time, the muscles will be in a state of tension and fatigue, and a slight strain and strain will occur over time, forming a sterile inflammatory lesion, and the inflammatory metabolites will stimulate the peripheral nerves around the area, causing a significant pain sensation. When standing, the muscles will be strained again due to the change of posture, resulting in further stimulation of the strain and injury and significant pain. In addition, as local inflammatory edema forms after the injury, these inflammatory edema will also produce further stimulation of the surrounding tissues and will cause significant pain.