Leg fossa also refers to popliteal fossa, the cause of pain involves knee injury, popliteal cyst, lumbar disc herniation and so on. 1. Knee injury: strenuous or excessive exercise, knee sprains, etc. may damage the popliteal ligaments, muscles, etc., and pain is caused by pulling the damaged area during activities. 2. Popliteal cyst: Inflammation of synovial membrane due to strain injury or aging of joints, and then secretion of tissue fluid to form cysts gradually, typical manifestations of popliteal fossa distension and pain, and there is a foreign body when touching. 3. Lumbar disc herniation: when the herniated disc compresses the sacral nerve root or lumbar nerve root, the lumbar pain will radiate to the popliteal fossa, resulting in popliteal fossa pain and lack of strength in the calf. There are many other causes of pain in the leg fossa, it is recommended to go to the hospital, improve the examination to clarify the root cause.