Back of the knee. Artwork of the anatomy of the popliteal fossa, the hollow at the back of the knee. At top left is the semimembranosus muscle (red) and top right is the biceps femoris muscle. They both flex the knee and rotate the tibia (shin bone). The muscle at bottom is the gastrocnemius (calf muscle), consisting of the inner medial head and outer lateral head. It flexes the foot at the ankle and raises the heel during walking. At centre are the popliteal vein (blue) and artery (red), a continuation of the femoral artery. The fibular (left) and tibial (centre) branches of the sciatic nerve (yellow) are also seen. Tendons, which attach muscle to bone are white.