The fossa between the thigh and calf is called the popliteal fossa, which is the back side wall of the knee joint. Inside the popliteal fossa are many tissues such as muscles, tendons, ligaments, fascia, lymph, arteries and veins, and nerves. The skin of the popliteal fossa is roughly the top layer of the popliteal fossa, and underneath the skin are soft tissues such as fat, muscle, and fascia, as well as important blood vessels and nerves, most of which are located in the deeper layers of the popliteal fossa and are not easily damaged. Most of the muscles and ligaments in the popliteal fossa control the flexion and extension of the lower limbs. When a person stands, the popliteal fossa is normally flat or slightly concave. If the popliteal fossa protrudes when standing, it may be due to frequent and heavy exercise or obesity, or it may be due to localized lesions of the knee joint, such as popliteal cysts, joint effusion, etc., as well as patients suffering from lumbosacral disorders can cause popliteal protrusion.