Origins, insertions, innervation and functions of the inner hip muscles.
Hello everyone! This is Joao from Kenhub, and welcome to another tutorial. On this one, I’m going to be talking about the inner hip muscles.
Without keeping you waiting, I’m going to show you here an image of the ventral view of the hip muscles or the inner hip muscles. Now, you can also see other muscles including the pelvic diaphragm or muscles of the pelvic diaphragm, and the muscles of the anterior thigh, and also the adductors. Now, let’s remove them and show you now, expose you completely the muscles of the inner hip.
Now, on this tutorial, we will be talking about the muscles of the inner hip, like I mentioned, which is a small group of three muscles which include the iliacus, the psoas major, and a relatively rare muscle known as the psoas minor. Now, sometimes, these muscles are also referred to as the dorsal hip muscles.
Now, let’s move on and before we do so, I want to clarify a point here on this slide. And the psoas major and the iliacus are usually referred to as a single muscle known as the iliopsoas. And those two muscles have different origin points, and their bellies and tendons merge into one and attach together on the same bony point. We’re going to look at that later on when we discuss the psoas major and the iliacus individually. But now you know that from now on, when you hear the word iliopsoas muscle, you know that we’re talking about a combination of two muscles.
Before we move on to our tutorial, I want to talk a little bit more about the iliopsoas. And to do so, I'm showing you now the image on the ventral view of the muscle and including here some blood vessels and nerves. Now, what you see here on this image is the psoas major on the subject’s right side… on the subject’s left side and your right side, and here, you see also the iliacus. Now, on the subject’s right side and on your left side on the screen, you see only one muscle, the iliacus, and we just removed the psoas major so we can expose here the structure known as the lumbar plexus.
Now, I wanted to use this image to show you here that the psoas major and also the iliacus, they will unify here on the lateral pelvis, and shortly, before this ligament here, which is known as the inguinal ligament, they will become then the iliopsoas muscle. And here, they pass below the inguinal ligament through the muscular lacuna together with the femoral nerve.
Now, both muscles are completely surrounded by iliac fascia. And the lumbar plexus lies dorsally from the psoas major muscle. So as you can see, we removed here the psoas major so we could expose the lumbar plexus for you. And you can see here, then, on your right side, the muscle is here and covering a bit of the lumbar plexus.
Now, medially from the psoas major, runs the sympathetic trunk, and also, you can clearly see here on this image that the genitofemoral nerve pierces through the psoas major as you can see here.
Time to start covering the muscles in a little bit more detail, starting off with the iliacus. We’re going to talk about the origin point for this muscle. And what you need to know is that it will arise from the iliac fossa on the wing of the ilium, as you can see here. So these are the origin points for the two iliacus muscles.
Now, in terms of the psoas major, the origin points for this muscle—there are several that you need to remember actually—and it originates from the lateral sides of the bodies of the vertebrae T12 to L4, and also, their respective intervertebral discs, T12 and L4. And it also arises from the coastal processes of L1 and L5, to L5.
Now, moving on, we’re going to talk about the insertion point for the iliopsoas. Like I mentioned, these two muscles, the iliacus and the psoas major, are going to come together and then insert into one common insertion point which is the lesser trochanter of the femur. And you can see here where these two muscles here are going to be inserting. In terms of the innervation of the iliopsoas, it is carried by the femoral nerve for the iliacus as well as direct branches of the lumbar plexus for the psoas major.
Now, it is time for us to talk about the different actions or functions of the iliopsoas. And the first thing that I need to highlight is this muscle is the strongest flexor of the hip joint. Now, it is important for walking, standing, and running. Now, altogether, the iliopsoas muscle plays a significant role in the movement and stabilization of the pelvis, but there are other functions that we need to highlight, including this one that is depicted here on this image on the right side of the screen. This is the flexion of your thigh—so when you move your thigh close to your belly.
Now, the other one is going to be, then, depicted here. It’s already seen on the sentence but depicted here on the image. This is lateral rotation at the hip joint. The other function that we see on the ilopsoas is now going to be depicted by this image here, and this is known as the lateral flexion of the lumbar spine. So when there is a unilateral contraction with the femur fixed, this will lead to a lateral flexion of the vertebral or the lumbar vertebral column. So the trunk will bend laterally to the same side of the contraction. So it’s going to either bend to the left side or to the right side depending on which muscle will be contracting. And the last function here on our list is going to be seen here on this image. It’s known as ventral flexion. And that is when you bend the trunk forward, lifting the trunk when lying down in supine position—so when you’re trying to do sit-ups.
And now, that we covered the functions, origin, insertions, and innervation of the iliopsoas, it is time for us to move on to the last muscle on our list, the psoas minor. And now I'm going to remove here this layer of muscles and showing and exposing the psoas minor which is a long, slim muscle of the hip joint that runs ventrally to the iliopsoas. As the muscle lies deep inside your abdomen, it is practically not possible to be felt from the outside.
And another important thing that I need to stress here is that the psoas minor is not part of the iliopsoas. So this might be object of confusion sometimes, so write this down on your notes. Another important thing to mention about the psoas minor is that it is an inconstant muscle, and it is not found in 40 - 70% of all people. You often find, instead of a thin… instead of this muscle, a thin ligament or broadening of the medial part of the iliopsoas.
Now, let’s talk about the origin points for the psoas minor, and it originates at the lateral surface of the bodies of the twelfth thoracic and first lumbar vertebrae and also their respective intervertebral discs. In terms of insertion point for this muscle, it is relatively long, as you see here, and it goes to insert on the iliopubic eminence and also the pectineal line of the pubic bone. Additionally, fibers of the insertion tendon are attached to the iliac fascia.
Now, it is time for us to cover the innervation of the psoas minor, and all you need to know is that this muscle is going to be innervated by direct branches of the lumbar plexus.
Let’s talk about the functions or actions associated to the psoas minor. And as an inner hip muscle, the psoas minor contributes to the stabilization of the pelvis and the hip joint. But you can also add here to this list lateral flexion. So when there is a unilateral contraction, the muscle will bend the lumbar vertebral column to the side of the contraction. The other one is going to be ventral flexion when there is a bilateral contraction. So when these two muscles are going to contract at the same time, it will bend it to the front, so like you’re doing sit-ups, and this is what is known as ventral flexion.
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