These muscles are called the vastus intermedius, the vastus medialis, the vastus lateralis, and the rectus femoris. The quadriceps are also known simply as the quads.When your quads contract, they straighten your leg at your knee joint. Since the quadriceps extend over the kneecap (patella), they also help to keep your kneecap in its proper position in a groove in the end of your thigh bone. One of your four quad muscles also crosses the hip joint and can assist in flexion, or bending up, your hip.
Click HERE to see our exercise of the month: Squats to help strengthen the Quadriceps