This image shows some of the thigh muscles of a dissected human male cadaver.  The longest muscle in the human body is the sartorius,  which results in flexion of the thigh and weak flexion at the knee joint.    Posterior to the sartorius, on the lateral surface of the thigh, is the tough and semitransparent fascia lata.  The anterior half of the thigh in the human contains the quadriceps group, which is involved primarily in the extension of the knee.  These muscles include the vastus lateralis, vastus medialis, vastus intermedius and rectus femoris. The medial surface of the thigh, posterior to the sartorius and quadriceps group is covered by the gracilis, which  adducts the thigh and flexes the leg.   Its  insertion is just inferior to the medial condyle of the tibia.