The obturator internus is a muscle located within the human pelvis. Deep to the gluteus maximus, it is situated among the external rotators of the hip, muscles like the quadratus femoris and the piriformis, on the posterior or rear side of the pelvis. Also, while involved in lateral or external rotation of the thigh as well as in hip adduction, this muscle is not a prime mover of the hip joint. In other words, it assists larger muscles of the hip and thigh in performing both of these actions.
Shaped like a narrow band, the obturator internus finds its origins on the pubis and ischium bones in the pelvis. The pubis is the more medial of the two lower bones of the pelvis, found closer to the pubic region, while the ischium is the more lateral of the two, situated nearer to the hip joint. These two curved bones meet in the middle to form a loop. The obturator internus arises from the perimeter of this loop. Specifically, it originates on the internal margins of the obturator foramen, which is the name for the hole inside of the loop, as well as on the obturator membrane, an expanse of fibrous tissue that covers most of the foramen.
From here the obturator internus crosses the pelvis laterally, or horizontally, posterior to the pelvic bones. Tapering as it approaches the hip joint, it inserts via several tendon-like sections on the medial aspect of the greater trochanter of the femur bone in the thigh. The greater trochanter is a bony surface on the posterior side of the neck of the femur. The obturator internus inserts just to the inside of the trochanter above the trochanteric fossa, a small cavity where it and three other hip muscles attach. En route to the femur, it passes through the lesser sciatic foramen, an opening behind the pelvic bones bordered by the sacrum in the base of the spine, the ischium bone in the pelvis, and the sacrotuberous and sacrospinous ligaments, both of which run between the sacrum and the ischium.
When contracting, the obturator internus can perform two actions: lateral rotation of the femur and abduction of the hip when flexed. During lateral or external rotation, contraction of the obturator externus turns the posterior surface of the femur medially, causing the hip to open and the leg to turn out like that of a ballet dancer. The action of the muscle during hip abduction is to pull the leg, when the hip is flexed, from in front of the body out to the side, as in stepping out of a car.