|Derivation||The largest (maximus) muscle of the buttock (gluteus).|
|Origin||Posterior, lateral surfaces of ilium,sacrum, and coccyx. (Also a number of deep ligaments on the pelvis.)|
|Insertion||Upper, posterior surface of the femur, and the iliotibial band.|
|Action||Straightens the thigh at the hip and draws it backwards.|
Gluteus maximus is a powerful muscle that forms most of the bulk of the buttocks. Viewed from the side its shape is familiarly round. From the back, it appears as an oblique, rounded rectangle.
It ducks between biceps femoris and vastus lateralis on its way to its insertion on the femur. It also attaches to the iliotibial band.
The illustration reveals that the muscle does not account for the full shape of the buttocks. There is a layer of fat and fasciae lying medially below the muscle that forms the gluteal cleft (the split between the buttocks) and makes the gluteal fold under the buttocks more or less horizontal in a straight standing position. This fold is an important line when establishing the shape of the buttocks. In practice, the line of the fold can often be treated as the lower border of the muscle, and drawn to the insertion on the back of the femur.
Two depressions corresponding to the posterior superior iliac spines of the pelvis and the top of the gluteal cleft form what is known as the sacral triangle. Apart from its being an important form in its own right, it indicates the position of the sacrum (which lies under it) and is invaluable for determining the angle of the whole pelvis.