The vastus intermedius is and extensive muscle that lies deep to all the other quadriceps bellies, adding muscular fullness under them. The quadriceps muscles consist of: the |rectus femoris|, the |vastus lateralis|, the vastus intermedius, and the |vastus medialis|. These muscles form a large fleshy mass that covers the front and sides of the |femur| (upper leg bone). They work together as the primary extensor of the knee.
