The armpit of the leg is the popliteal fossa, and the common causes of popliteal pain may be popliteal cysts, local soft tissue injuries, and osteoarthritis of the knee. 1. Popliteal cyst: the most common cause is meniscus injury, the joint fluid of the knee joint penetrates into the popliteal fossa through the joint capsule, forming a cystic swelling, the cyst compresses the peripheral nerves and produces numbness and pain. 2. Local soft tissue injury: soft tissue injury in the popliteal fossa is mainly caused by the injury of semitendinosus, semimembranosus, triceps and other tendons, which triggers the swelling and pain in the popliteal fossa, and seriously affects the knee flexion and extension activities. 3. Osteoarthritis of the knee joint: more common in middle-aged and elderly people, the knee joint often suffered serious trauma or long-term strain, triggering swelling, pain, stiffness, deformation of the knee joint, activities are limited, but also popliteal fossa pain performance. Leg armpit pain may also have other causes, it is recommended to go to the hospital in time, improve the examination to clarify the cause of the disease, give targeted treatment or treatment.