The quadriceps, or quads, are a group of four (4) muscles in your thigh. These muscles are the vastus intermedius, the vastus medialis, the vastus lateralis, and the rectus femoris. Together, these muscles work to extend and flex the leg, and are one of the most powerful muscle groups in the body. Because they are so involved in everyday movements such as walking, standing, and running, they are prone to injury. Some of the most common quad-related injuries are tendonitis, cramps, strains, tears, and pulls.