Deep Dive into the Anatomy of the Hip Flexor Muscles

For those of you that love to get deep into the details, you will love this section. As an anatomy geek myself, I can’t help but get excited about this topic, especially when digging into arguably the most important muscle group in the body.


What is the origin, insertion, and nerve supply of the iliopsoas? 


The origin of a muscle is the location where the muscle attaches closest to the center of the body. The insertion is the attachment point furthest away from the center of the body. The origin of the iliacus is the inside surface of the pelvic bone (the iliac crest and iliac fossa on the ilium) and the top edge of the tailbone (the ala of the sacrum). The insertion of the iliacus muscle, along with the psoas, is located at the inside of the femur near the groin (the lesser trochanter on the femur). The origin of the psoas is the front (anterior) surface of the lower vertebrae (T12-L5) and the bottom of the transverse processes of those same lower vertebrae. The insertion of the psoas is the same as the iliacus, on the lesser trochanter.

The origin and insertion of the psoas

The origin and insertion of the iliacus

The iliacus and psoas blend together to form the iliopsoas tendon where they both attach at the lesser trochanter. The iliopsoas tendon translates the force produced by the muscle belly of the iliacus and psoas themselves, moving the femur bone relative to the pelvis and low back. As you can see, the iliacus crosses 2 joints (the sacroiliac joint and the hip joint) and the psoas crosses 3 joints (lumbar vertebrae, sacroiliac joint, and hip joint). Oftentimes the iliopsoas is only considered a 2 joint muscle because the sacroiliac joint is not counted, no respect for the SI.

 The iliacus and psoas technically cross 3 joints

Nerves coming off the spinal cord create a complex network that shares information with the muscles, skin, and other tissues in the area. Special nerves head straight for the iliacus by exiting the spine at L2-L4, joining to form the Femoral Nerve, and innervating the iliacus. Nerves exiting the spine at L1-L3 join to form the Ventral Primary Rami and innervate the psoas. Interestingly, the signals from the brain to these two muscles are distinctly different, showing they are acting independent of each other and should not be considered one muscle. Because they are often called the “hip flexor” or iliopsoas, the iliacus and psoas are often considered one in the same when they are each distinctly different in their action, purpose, and location.

What else is connected to the psoas and iliacus?

Because everything in the body is tied together by connective tissue like fascia, ligaments, tendons, and joint capsules, the iliacus and psoas are connected to the tissues around them. This includes the digestive system, nerves, urinary system, lymph system, and reproductive system.

The iliacus and psoas are located adjacent to many of the abdominal structures

Fascia is a specialized connective tissue that is continuous from head to toe. The purpose of fascia is to hold everything in place. If we didn’t have fascia our organs would end up in the wrong place and our muscles would be sagging off of our bones! Not very attractive. The fascia that is connected to the iliacus and psoas is also directly connected to the pelvic floor and diaphragm. This gives a “C” shape of connective tissue that wraps along the pelvic floor, follows the iliacus and psoas up the back of the abdomen, and then curls under the ribs along the diaphragm. Because of this connection, tightness in the psoas or iliacus will pull on the diaphragm and pelvic floor muscles, nerves, and organs.

Fascia creates a “C” shaped connection between the pelvic floor, iliopsoas and diaphragm

The iliacus is of particular interest to the pelvic floor itself. Issues with urination, sexual function, and pelvic pain are commonly associated with tightness in the iliacus. You can see how intimately connected these structures are in location and with their fascial connection. Similarly, when the iliacus is tight, it rotates the pelvic bones which directly affect the strain of the nerves and muscles in the pelvic floor and hip regions, often manifesting in chronic tension and nerve pain

The iliacus and its intimate connection to the pelvic floor

What does the iliacus and psoas do?


The function of the iliopsoas varies depending on the position and activity of the body. Although these muscles are often referred to together as the hip flexor or iliopsoas, their action and function are different. This is an underappreciated and very important point. They both contribute to hip flexion motion but their primary job is to hold together the hip, pelvis, and lower back area.


What does the iliacus do?

Primarily, the iliacus stabilizes the hip joint and the SI (sacroiliac) joint. Because it is relatively short and strong and crosses the two joints of the hip and the sacroiliac joint, it acts as more of a stabilizer, keeping those two joints aligned and strong regardless if the body is stationary or moving. Long muscles have good lever arms for moving bones well. A good example of this is the quadricep that goes from the hip to below the knee. Short muscles are built to hold parts in place. This is what the iliacus was born to do, stabilize the hip and SI joint. Any situation that challenges these 2 joints will lead to a challenge to the iliacus muscle and possibly the development of chronic tightness. 

Why does my leg rotate out when I lay down? Why do I need a pillow under my knees when sleeping?  

A tight hip twists the leg. The action of the iliacus when it moves the femur bone is

flexion and external rotation of the hip. Because of this, when you have a tight iliacus and your pelvis is stationary like when you are lying down, your leg will rotate outward. Tightness here may also put your hip into a little bit of flexion when you lay down, making it hard to lie on your back without a pillow under your knees. These are two signs that you have chronic holding patterns in this muscle. If a muscle is tight laying down, it’s tight all the time, during everything you do. It doesn’t take much imagination to see how that can cause issues with performance and pain. 

Why is my pelvis rotated? Why does my hip go out?


A tight hip twists the core. When the leg is fixed instead of the pelvis, like in standing for example, the pull of the iliacus can cause a rotation of the pelvic bone forward, changing the alignment of the pelvis, hip joint, femur, and the rest of the leg. This is called an anterior rotation and this pulling motion works its way up to the spine as well. This is a very common sign of a tight iliacus and can get stuck that way and affect your alignment of the entire body, leading to pain and poor muscle strength. 

The iliacus rotates the pelvis forward and affect the entire body

Does the iliacus move the spine? Is the iliacus attached to the spine?

When the leg is stationary, the iliacus will help bend the spine to the same side (ipsilateral side bending) and rotates the pelvis forward when you go from laying down to long sitting, contributing to spinal flexion in this situation. Although the iliacus does not directly attach to the spine, its connection to the pelvis and how it moves that bone, affects the motion of the spine.

What does the psoas do?

Similar to the iliacus, the psoas stabilizes the hip joint and SI joint but, due to its connection to the lumbar spine, is involved much more than the iliacus in stabilizing the lower back. Its close connection to the lumbar vertebrae and its attachment to the front and back of each vertebrae give it the perfect advantage to help hold that lumbar spine in place. When there is any issue in the low back, the psoas grabs on for dear life to hold it all together, easily developing into chronic tightness. Its complex attachments at the lumbar spine also contribute to motion of the spine such as rotation, sidebending, flexion and extension, but primarily, its job at the lumbar spine is to stabilize.

Why does my back arch so much?

Some people experience what is called excessive lumbar lordosis where their lower back arches too much. Because of the long length of the psoas, attaching at the top of the lumbar spine and running all the way to the inside of the femur, it does a good job of extending the low back and creating an anterior pelvic tilt. This motion is best seen when sitting when you create extension in your spine to sit with a lumbar curve. Slouching in your chair relaxes the psoas, sitting in good posture activates the psoas (and iliacus). People who have psoas tension tend to have an excess arch in their spine when they stand or lay down as well.

A tight psoas causes excess arch in the low back (lordosis)

Does the psoas move the hip joint?

Like the iliacus, the psoas creates flexion and hip external rotation at the hip joint. This can also contribute to one leg rotating outward when standing or laying down, a tight psoas or iliacus could be the culprit along with the bony structure of your hip and pelvis.

What muscles oppose the iliopsoas (antagonists)?

Antagonists is a name for muscles that does the opposite motion. In this example, the opposing muscles to the hip flexors are those that mostly produce hip extension (kicking the leg behind you). These antagonists are not necessarily bad, they just have the opposite job of the hip flexors. Since the hip flexors are on the front of the body to pull the leg forward, the antagonists are mostly in the back of the body.

The tug-of-war of opposing muscle groups is how the body moves so elegantly. One group of muscles pulls one way while the opposing muscles pull the opposite

direction, both attempting to keep the bones in a precise location in each moment. When the iliacus and psoas are pulling too much, the opposing muscles activate to keep the iliopsoas from winning. 

The tug of war played by the iliopsoas

The gluteus maximus and minimus (often called the glute) help pull the leg backward. Quadratus lumborum and low back muscles get cramped and tight when the iliopsoas is tight. The hamstring muscle, attaching directly to the pelvis, plays a mean game of tug-of-war with a tight iliacus. Joining the game, deeper in the pelvis, lives the piriformis muscle. The sciatic nerve runs right underneath it, so when piriformis plays tug-of-war, that nerve is affected.

Antagonists of the iliopsoas on the back of the body

The obturator internus muscle is another undertreated and undiscovered muscle deep in the pelvis that is quite tight in a lot of people who have tight iliopsoas. This muscle is really close to the back part of the hip joint, similar to how the iliopsoas is close to the front part of the hip joint. They are definitely tugging on each other. The obturator internus and the iliacus are often tight together with issues with the hip joint and the pelvic floor. Obturator internus is part of a larger group of muscles called the deep hip rotators. 

Obturator internus is positioned directly opposite the iliacus.

Take a moment to explore the antagonist on the back of your own body. The part of the thigh bone that is on the outside of the hip (greater trochanter) is where a lot of the antagonists attach, like the deep hip rotators, glute, and piriformis. This bone can hurt when those muscles are angry. Following the hip crests to the back of the pelvis, you find two bony knobs that may look like dimples. This is where the pelvic bone (ilium) and the tailbone (sacrum) connect. This is the SI (sacroiliac) joint, near where the other end of the piriformis attaches. The lower back starts in the center between those two bony points. To the side of the tailbone in the soft fleshy part is the bulk of the antagonist muscles. On the back of the thigh is the hamstring muscle. If you sit down on a chair and you put your hands under your bottom you will find two bony points, the “sit” bones, where the hamstring attaches. Moving your fingertips to the inside surface of the sit bones, you’ll find the obturator internus and the space in between the sit bones is the pelvic floor, full of muscles waiting to get tight.

Find some of these opposing muscles on yourself

What muscles work with the iliopsoas (synergists)?

There are few other muscles that have a similar job bringing the leg forward with the motion of hip flexion. Muscles that work together are called synergists. When one muscle is overused and tight, it’s not uncommon for the synergists to also be sore and tight.

Pectineus (peck-ti-knee-us) is in the inner thigh. Sartorius goes from the outer hip to the inner knee. Rectus femoris is part of our quadricep, the big muscle on the front of the thigh. Tensor fasciae latae (ten-sir fash-ha lot-a), one of my all-time favorite names for a muscle, attaches to the IT (iliotibial) band on the outside of your leg. Part of the gluteus medius, located on the outside of the hip, also supports the iliopsoas. All of these muscles are hip flexors and support the iliacus and psoas in that motion. 

Other major hip flexors acting as Sidekicks to the iliopsoas (gluteus medius and sartorius not shown)

Want to learn more? Be sure to check out my best selling book Tight Hip, Twisted Core – The Key to Unresolved Pain or order the Hip Hook to release the tension in your very own hip flexor. It may be the very cause of your pain.


What muscles are synergists to the iliopsoas?
Does the quadratus lumborum oppose the iliopsoas?
What muscles oppose the iliopsoas?
Why does my hip go out? Why is my pelvis rotated?
Why does my back arch when I lay down?
Why does my leg rotate out when lying down?
What is the function of the psoas?
What is the function of the iliacus?
What is the function of the iliopsoas?
Do the iliacus and psoas attach to the organs?
Does the psoas attach to the diaphragm?
Is the iliopsoas a 2 joint muscle?
Where does the iliopsoas tendon attach?
What is the innervation (nerve supply) of the psoas?
What is the origin and insertion of the psoas?
What is the innervation (nerve supply) of the iliacus?
What is the origin and insertion of the iliacus?