Nerve supply of the gemelli
The gemelli (Latin for twins) are a pair of narrow deep muscles of the hip, and each one takes its nerve from the muscle below it.
Remember
Each gemellus steals its nerve from the muscle below it: the superior gemellus takes the nerve to obturator internus (the muscle whose tendon lies between the twins), and the inferior gemellus takes the nerve to quadratus femoris (the flat, square muscle at the bottom of the deep group).
Memory aid
Thieving twins
Each twin steals its nerve from the muscle below it, which tells you which nerve belongs to which gemellus.
The gemelli are two narrow muscles in the deep gluteal region (the layer of short muscles lying beneath the large gluteal muscles of the buttock). They sit either side of the tendon of obturator internus, with the superior gemellus above it and the inferior gemellus below it.
They are nicknamed the thieving twins because each one steals its nerve supply from the muscle below it. The superior gemellus therefore takes the nerve to obturator internus. The inferior gemellus lies below that tendon, so it takes the nerve to quadratus femoris, the nerve of the muscle immediately below it.
The superior gemellus arises from the ischial spine (a bony point at the back of the pelvis), while the inferior gemellus arises from the ischial tuberosity (the bone you sit on). Both attach to the greater trochanter (the bony prominence at the top of the outer thigh bone). Both gemelli assist obturator internus: they laterally rotate the extended thigh (turn it outwards) and abduct the flexed thigh (move it away from the midline).
The gluteal nerves are the usual trap. The superior and inferior gluteal nerves supply the larger, more superficial gluteal muscles, and they do not supply the small deep rotators. Injury to the nerve to quadratus femoris weakens two lateral rotators, the inferior gemellus and quadratus femoris itself, whereas gluteus maximus keeps its strength, because the inferior gluteal nerve supplies it.
Key facts
- Superior gemellus
- supplied by the nerve to obturator internus, and it arises from the ischial spine.
- Inferior gemellus
- supplied by the nerve to quadratus femoris, and it arises from the ischial tuberosity.
- Rule
- each gemellus takes the nerve of the muscle below it.
- Action
- both gemelli laterally rotate the extended thigh and abduct the flexed thigh.
- Gluteal nerves
- the superior and inferior gluteal nerves supply the larger superficial gluteal muscles, not the gemelli.
- Nerve injury
- injury to the nerve to quadratus femoris weakens two lateral rotators, the inferior gemellus and quadratus femoris, while gluteus maximus keeps its strength.
Exam traps
- The nerve to obturator internus is the classic swap for the inferior gemellus: it supplies the superior gemellus, whereas the inferior gemellus takes a different nerve.
- Weakness limited to a deep lateral rotator does not point to the inferior gluteal nerve, because that nerve supplies gluteus maximus, not the deep rotators.
- The obturator nerve is not the nerve to obturator internus: despite the shared name, it supplies no muscle of the deep gluteal group.
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