The major muscles in the human upper leg are in two groups: the hamstrings and the quadriceps. The semitendinosus, semimembranosus and biceps femoris muscles comprise the hamstrings muscle group, while the rectus femoris, vastus lateralis, vastus intermedius and vastus medialis muscles make up the quadriceps.
The hamstring muscles cover the back of the thigh and govern hip movement and knee flexion, assisting in walking, running, jumping and many other daily activities. The quadriceps group covers the front of the thigh and functions as an extensor for the knee and lower leg. The quadriceps are some of the leanest and strongest muscles in the entire human body.