The buttocks are two rounded portions of the exterior anatomy of most mammals, located on the posterior of the pelvic region. In humans, the buttocks are located between the lower back and the perineum. They are composed of a layer of exterior skin and underlying subcutaneous fat superimposed on a left and right gluteus maximus and gluteus medius muscles. The two gluteus maximus muscles are the largest muscles in the human body. They are responsible for movements such as straightening the body into the upright (standing) posture when it is bent at the waist; maintaining the body in the upright posture by keeping the hip joints extended; and propelling the body forward via further leg (hip) extension when walking or running.