So what are these mysterious, hard-gripping, pose-disrupting muscles? The buttock muscles can be divided into two layers: the more superficial one is the gluteus maximus, which, when well developed, forms the rounded shape of the buttocks. It originates on the sacrum at the base of the spine and nearby pelvis, and then runs diagonally down and across the buttocks to insert on the outer upper femur (thighbone). When it contracts, it extends the hip, which pulls the femur into line with the torso. For example, the gluteus maximus contracts when you stand up.
In yoga, the gluteus maximus helps perform a similar job in partnership with the hamstrings when you move from a standing forward bend to standing upright. The gluteus maximus is also a strong external rotator of the hip, which, when you're standing, turns the knees outward.
The second and deeper layer of buttock muscles is made up of the six deep rotators. The piriformis is the best known, but this group also includes the internal and external obturators, the superior and inferior gemelli, and the quadratus femoris. They originate on the sacrum and the ischial tuberosities (sitting bones), and then run diagonally in a fan shape across the deep buttocks to insert on the back of the upper femur on the greater trochanter.!--page-->