The gluteus maximus is a very large, fleshy muscle at the back of the hip (the one you sit on).
The gluteus maximus originates on the outer surface of the crest of the ilium, the fascia of the lumbar area, and the sides of the coccyx.
It inserts on a broad line about four inches high on the posterior side of the femur and the iliotibial tract of the fascia latae.
The gluteus maximus is involved mainly in extension and outward rotation of the hip joint. The upper fibers also assist in hip joint abduction, and the lower fibers assist in hip joint adduction.