Inferior gemellus muscle

Origin and insertion

The small narrow inferior gemellus muscle extends between the coxal bone and femur.

It originates from the lateral surface (upper part) of ischial tuberosity, inferior to the groove occupied by the tendon of the obturator internus muscle.

It courses horizontally in a lateral direction, merging with the inferior border of the tendon of obturator internus. Inferior gemellus then inserts into the medial surface of the greater trochanter of femur via this tendon. 

Relations

Inferior gemellus forms part of the triceps coxae muscle complex, which is located in the deep layer of the gluteal region between the piriformis and quadratus femoris muscles. Inferior gemellus is overlaid superficially by the gluteus maximus muscle, while the hip joint is located deep, or anterior, to it. The nerve to quadratus femoris descends deep to inferior gemellus after exiting the pelvis through the greater sciatic foramen. The sciatic nerve, posterior femoral cutaneous nerve and inferior gluteal artery course superficially, or posterior, to inferior gemellus.

After obturator internus exits the pelvis through the lesser sciatic foramen, inferior gemellus forms a muscular canal for its tendon. Inferior gemellus travels inferior to the tendon initially, and then passes anterior to it as they approach the insertion point on the femur.

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Innervation

Inferior gemellus receives innervation from the nerve to quadratus femoris which is a branch of the sacral plexus. It arises from L4/5 and S1 spinal nerves. 

Blood supply

Inferior gemellus muscle receives arterial blood from the medial circumflex femoral artery, which arises from the deep femoral artery. After vascularizing the muscle, the medial circumflex femoral artery continues further to supply the neck of femur.

Functions

Inferior gemellus is an accessory reinforcement to obturator internus, aiding it in its actions. This aspect is necessary because obturator internus loses a proportion of its power once it leaves the pelvis and turns almost ninety degrees when crossing the lesser sciatic foramen.  

Inferior gemellus muscle acts on the hip joint, causing two major movements. When the thigh is in an anatomical position, it pulls the greater trochanter of femur posteriorly, around the bone’s longitudinal axis, causing external (lateral) rotation of the thigh. When the hip, and hence the thigh, are flexed, inferior gemellus pulls the upper femur medially. This action is compensated by the lower femur moving laterally, resulting in thigh abduction. Inferior gemellus also reinforces the integrity of the hip joint, stabilizing the head of femur inside the acetabulum. 

The combination of thigh external rotation and abduction from a flexed hip has important functions in our everyday lives. These actions allow you to move sideways when seated and to swing your lower extremity sideways when stepping outside of a car. They stabilize the trunk in a sitting position during side to side jerking and help with squatting and crawling.  

For more information about the gemelli muscles and their movements, take a look below: