Muscle · Glutes

Obturator internus

Musculus obturator internus

A fan of muscle inside the pelvis that covers the obturator foramen, then does something unusual: its tendon makes a right-angle turn around the lesser sciatic notch — using the bone as a pulley, complete with a bursa — and leaves the pelvis to reach the greater trochanter. That turn is why it rotates the hip outward from a position no other muscle occupies. With the two gemelli attached to either side of its tendon it forms the triceps coxae. The founder's system groups it with the deep hip rotators and assigns them all one job: femoral head stability.