Video: Iliopsoas muscle (3D)

Who doesn’t love getting a great deal? All of you coupon cutters out there know what I’m talking about, right? And the savviest of you shoppers know that two-for-one deals are among the best bargains. Believe it or not, anatomy sometimes presents its own kind of special deals also. Two muscles for the price of one? Sounds like madness, right? Well, today, we’ll be focusing on a muscle which cashes in on one of these two-for-one deals, since it is actually comprised of two smaller muscles. This bargain of a muscle is called the iliopsoas, and today, we’ll be talking about all of its functions.

Before looking at how this muscle functions as one whole unit, let’s first break the iliopsoas down into its two components, starting with the iliacus.

As its name suggests, it originates on this large broad bony feature called the iliac fossa of the hip bone. From here, it tapers as it travels inferiorly deep to the inguinal ligament, where it inserts at the lesser trochanter of the femur located at the posteromedial aspect of the proximal femur. But as we’ve seen, this is only half of the picture for the iliopsoas, with the other half comprised of the psoas major muscle.

The psoas major muscle originates medial to the iliacus or closer to the midline. Specifically, it usually arises from the vertebral bodies of T12 to L4 and their adjacent intervertebral discs as well as the transverse processes of all five lumbar vertebrae. Then the psoas major muscle travels inferiorly joining the iliacus before inserting at the lesser trochanter of the femur. So even though the iliopsoas has several origins through both the iliacus and psoas major, it has one common point of insertion.

There is one small additional muscle that’s worth mentioning here, and that’s the psoas minor. Although this muscle does not contribute to the formation of the iliopsoas, it does help assist with some of the actions of the iliopsoas that we'll look at later in this video.

Like its big brother, the psoas minor originates from vertebral bodies, but this little guy only attaches to the vertebral bodies of T12 and L1. It then travels inferiorly to insert onto the pectineal line of the pubis and this is important to keep in mind for later on. Since the psoas minor does not extend to the lower limb, it won't play any role in the actions of the lower limb that the iliopsoas performs. And now you may be wondering, but what joints does this muscle act on? Great question. Let’s check it out.

Lucky for us, this dichotomous muscle is going to perform all of its actions on one primary joint, which is the acetabulofemoral joint, more often referred to as the hip joint. Additionally, we'll also see how this muscle can manipulate the lumbar spine through the attachment of the psoas major.

We have one last area to review before delving into the actions, and that's innervation. Both of the components of iliopsoas receive separate innervation before uniting, so we'll look at each muscle in isolation starting with the iliacus.

The iliacus muscle receives its innervation via the femoral nerve. Remember that the femoral nerve is comprised of nerve root levels L2 to L4 – a detail which might come in handy for your next anatomy exam. The psoas major, meanwhile, is innervated by direct branches of the lumbar plexus, specifically, the anterior ventral rami of spinal nerves L1 to L3.

Brilliant! Now that we're experts on the morphology of the iliopsoas muscle, let's see what it's capable of.

Let’s begin with the action that this muscle is most well-known for, which is a flexion of the thigh at the hip joint. In fact, the iliopsoas is most powerful thigh flexor. For this reason, it plays an important role in the gait cycle. Specifically, by flexing the thigh, the iliopsoas helps to lift the thigh and start the forward movement of the limb through what's called the swing phase of the gait cycle. Thanks to the iliopsoas, you're able to speed walk your way right on into class.

For our next action, we're actually going to have the model start by laying down in the supine position. In this way, we'll see how the iliopsoas and psoas minor is involved with flexion of the trunk at both the hip joint and the lumbar spine, which is what occurs when doing a sit-up. So next time you're pushing your way through a set of crunches, don't just give your abs all the glory, it's actually your psoas major and minor doing most of the work.

For the third section on the iliopsoas, we will return to our initial view. This will give us a good look at this muscle’s ability to perform external rotation which, of course, also occurs at the hip joint. Remember, external or lateral rotation of the hip joint is when the lower limb rotates laterally or away from the midline. Our coupon cutting friend here just saved a boatload at the shop and is in the middle of her happy dance. Notice that every time her knees go out laterally, she is activating her iliopsoas to laterally rotate the femur at the hip joint.

And with that, we come to our fourth and final action of the iliopsoas muscle – stabilization of the trunk and vertebral column. This is particularly important in helping to maintain good posture. With bilateral contraction, the psoas major, in particular, helps keep the trunk in place by pulling forward and down on the lumbar vertebrae reinforcing the lordotic curvature. Additionally, this helps keep the pelvis in its neutral position which is tilting forward at thirty degrees. If the iliopsoas were to weaken, the lordotic curvature can decrease and the pelvis would tilt more posteriorly flattening out the lumbar spine. This is known as swayback posture, and it can be harmful to the spine and hips as it puts additional pressure on these joints.

If we start in a seated position, we can also look at how the trunk is stabilized with a unilateral contraction of the psoas major. This movement also gets a little boost from the psoas minor, and this one is important so let's watch that again. Notice how the lumbar spine is being pulled towards the midline? In this way, the iliopsoas is very important for returning the spine to the neutral position keeping us upright.

And that's it! We’ve made it through all four functions of the iliopsoas muscle. We've learned a lot, so let's take a quick second to review.

We started with the primary function of the iliopsoas, which was flexion of the thigh at the hip joint. We then saw how the iliopsoas is able to perform flexion of the trunk at the hip joint and the lumbar spine. Next up was external rotation occurring at the hip joint, and finally, we learned how the iliopsoas works to stabilize the trunk and vertebral column by preventing hyperextension of the trunk at the hip joint with bilateral contraction as well as how unilateral contraction helps to keep the lumbar spine upright when sitting.

And just like that, we've reached the end. I hope you enjoyed learning with Kenhub today. See you next time.