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The obturator externus is a muscle located within the human pelvis. Though it is situated just inferior to or beneath the external rotators of the hip, muscles like the quadratus femoris and the piriformis, unlike these it is found not on the back of the pelvis but on the front side. 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.
Triangular in shape, the obturator externus finds its origins along a length of 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 known as the obturator foramen; the obturator externus arises from within this loop. Specifically, it originates on the medial or inner side of the foramen on the superior and inferior rami of the pubis as well as on the inferior ramus of the ischium.
From here the obturator externus crosses the pelvis laterally, or horizontally, running anteriorly to the pelvic bones. With fibers converging and twisting slightly and the muscle narrowing as it runs toward the hip joint, it inserts via a tendon alongside 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 externus inserts just medially or to the inside of the trochanter at the trochanteric fossa, a small cavity where it and three other hip muscles attach.
This particular hip muscle can perform two actions when contracting: lateral rotation of the femur and hip adduction, or pulling the leg inward toward the midline of the body from a side-raised position. 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 adduction is to pull horizontally on the femur, bringing the leg inward as in the jumping-together phase of a jumping jack.
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