The gemellus superior and gemellus inferior are two small, strap-like muscles flanking the obturator internus tendon as it exits the lesser sciatic foramen, effectively functioning as accessory heads of the obturator internus. Together with the piriformis, obturator externus, obturator internus, and quadratus femoris they form the six deep external rotators of the hip, a group critical for femoral head stability and controlled hip rotation.
| Origin | Ischial spine; Upper part of the ischial tuberosity |
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| Insertion | Both insert into the medial surface of the greater trochanter via the tendon of the obturator internus |
| Nerve Supply | Superior gemellus: nerve to the obturator internus (L5, S1); Inferior gemellus: nerve to the quadratus femoris (L4, L5) |
| Blood Supply | Inferior gluteal artery |
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They assist the obturator internus in external rotation and help stabilise the femoral head during single-leg activities by compressing it into the acetabulum from the posterior aspect.
The gemelli are rarely isolated clinically but are involved in the overall deep gluteal syndrome and are assessed as part of the deep external rotator group. Injury to the gemelli is associated with hamstring proximal avulsion injuries and posterior hip dislocation. They are identifiable individually on MRI but clinically assessed as a group with the other short external rotators.
The gemelli are not individually palpable but contribute to the muscle mass felt with deep pressure in the posterior hip between the ischial tuberosity and greater trochanter during resisted external rotation.
Injury to one or more of the short external rotators including the gemelli, producing deep posterior hip pain reproduced by resisted external rotation and passive internal rotation stretching.
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