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Functions and anatomy of the gemelli muscles shown with 3D model animation.
Today, we’re going to be looking at a little movement which means a lot in the world of martial arts, but hardly ever gets any attention. Something that will definitely help you in mastering that perfect sidekick. As you can see, this kick involves pivoting or rotating your back leg outwards to maintain balance while delivering the all-powerful blow. Today, we’re going to talk about two muscles which help you do just this by causing external or lateral rotation of the hip joint.
Studying for exams might make you feel like you’re having your butt kicked, but don’t worry, we’re here to help with this tutorial on the functions of the gemelli muscles.
First, we’re going to begin by familiarizing ourselves with the anatomy of these two small muscles. As we can see from our awesome 3D model here, the gemelli are a pair of parallel thin muscles that are associated with the upper and lower borders of the tendon of the obturator internus muscle, found posterior to the hip joint.
So above the tendon, we have the superior gemellus, and below it, we find the inferior gemellus, which work together to perform several functions with the obturator internus muscle. Funnily enough, the term gemellus in Latin means “little twin” so our gemelli are twins performing the same functions.
Together with the obturator internus, these muscles form the triceps coxae, which act on the hip joint. In a larger context, the gemelli muscles are part of a group of muscles known as the external rotators of the thigh or the hip joint, which gives us a clue to their function, but more on that later.
In order to fully understand the functions that these muscles perform, we have to look at the bones that they attach to and the joints that they cross.
Let’s start with the superior gemellus. If we zoom in a little bit closer, we can see that this muscle originates from the external surface of the ischial spine. It then merges along the length of the superior border of the obturator internus tendon before inserting onto the medial surface of the greater trochanter of the femur, which is this bony protuberance you can see here.
Okay, let’s move on now to the inferior gemellus. This muscle originates from the upper aspect of the ischial tuberosity which is the bony prominence that you sit on. The inferior gemellus also attaches along the length of the inferior border of the obturator internus tendon before inserting onto the medical surface of the greater trochanter. As the gemelli originates from the ischium of the hip bone and insert at the greater trochanter of the femur, they cross the hip joint. This means that the superior and inferior gemelli act on or perform movements at the hip joint.
So we’ve determined that the gemelli muscles act on the hip joint, but in order to carry out their functions, they need a nerve to tell them what to do and when. Although the gemelli muscles share a common name, they do not share a common innervation. The superior gemellus is innervated by the nerve to the obturator internus, which is formed by the L5, the S1, and S2 nerve roots of the sacral plexus. On the other hand, the inferior gemellus is innervated by the nerve to the quadratus femoris, which is formed by the L4, L5, and S1 nerve roots of the sacral plexus. This pattern of innervation nevertheless reflects their close association with the other deep muscles of the hip joint.
Finally, we can get to the functions of the gemelli muscles. As the gemelli muscles are part of the external rotators of the hip joint, it should come as no surprise that the first movement we’re going to talk about is external or lateral rotation of the thigh occurring, of course, at the hip joint.
If we look at our animation of this movement, we can see as the gemelli contracts, the femur is rotated outwards away from the midline of the body. Let’s look at it once again, but this time beginning with the hip in an internally rotated position and then externally rotating back to the neutral or anatomical position. This movement also rotates the foot outwards or away from the midline of the body, just like you can see here as our friend delivers a powerful sidekick just like we saw at the beginning of this video.
Together with the other external rotators of the hip, the gemelli are important when walking. Specifically, they produce external rotation of the hip joint in the early swing phase of gait.
The second movement of the gemelli muscles is abduction of the flexed thigh, which occurs at the hip joint in a flexed position. If we look at the movement once again, we can see that abduction describes the movement of the thigh away from the midline of the body. Last, but not least, the gemelli muscle also helps to stabilize the hip joint by supporting the head of the femur within the acetabulum. Strength and mobility of muscles, like the gemelli muscles, leads to decreased pressure and wear and tear on the hip joint, which prevents degradation and injury.
And with that, you’ll be pleased to hear that we’re almost finished our tutorial on the functions of the gemelli muscles. But before you go, let’s summarize what we covered today.
The first function of the gemelli muscles we saw was external rotation of the thigh at the hip joint, which involves rotation of the thigh outwards away from the midline of the body. The second function of these muscles was abduction of the thigh at the hip joint in a flexed position, which describes the movement of the thigh away from the midline of the body. The third and final function of the gemelli muscles was stabilizing the hip joint by supporting the head of the femur within the acetabulum.
And that’s us finished. Thanks for watching and feel free to check out our other 3D muscle function videos and lots more at kenhub.com. See you next time and happy studying!