The causes of pain in the posterior fossa of the leg are as follows: 1, popliteal nerve injury: often caused when the leg is stretched, such as the elderly leg press, if the knee joint is overly straightened, it will cause the posterior popliteal nerve to be strained, resulting in a strain on the popliteal nerve, which is mainly manifested as pain in the posterior aspect of the knee joint. 2, popliteal cyst: mainly due to the flow of joint fluid from the weakness of the posterior joint capsule It is a lump that accumulates at the back of the popliteal fossa. After regular walking and straining, the cyst will gradually become larger and accumulate at the back of the knee joint, and you will feel swelling, pain and swelling. If a popliteal cyst develops, arthroscopic surgery can be performed to remove the cyst and a compression bandage can be applied to prevent recurrence of the cyst.