The thigh muscles are a group of four muscles located in the upper leg. These muscles include the quads (quadriceps femoris), recti (rectus femoris), vasti (vastus medialis, vastus lateralis, and vastus intermedius) and quadriceps. The quads are the largest muscle group in the thigh and are responsible for extending the knee joint. The recti is a single muscle that runs from the hip to the knee and is responsible for flexing the knee joint. The vasti are three muscles located on the front of the thigh and are responsible for extending the knee joint. The quadriceps is a single muscle located on the front of the thigh and is responsible for extending the knee joint. Together, these four muscles are responsible for the movement of the thigh.