Jun 16, 2020

Deep Dive into the Anatomy of the Hip Flexor Muscles

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. You may want to also explore these topics:

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.

anatomy illustration of the psoas muscle
Origin and insertion of the psoas
anatomy illustration of the iliacus muscle
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). Often times the iliopsoas is only considered a 2 joint muscle because the sacroiliac joint is not counted, no respect for the SI.

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 independently 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.

Summary FAQs:

Q: What is the origin and insertion of the iliacus?

Origin - iliac crest and iliac fossa on the ilium bone

Insertion - lesser trochanter on femur

Q: What is the innervation (nerve supply) of the iliacus?

Femoral Nerve (L2-L4)

Q: What is the origin and insertion of the psoas?

Origin – anterior surface and transverse processes of T12-L5 vertebrae

Insertion - lesser trochanter on femur

Q: What is the innervation (nerve supply) of the psoas?

Ventral Primary Rami (L1-L3)

Q: Where does the iliopsoas tendon attach?

It wraps around the inner thigh and attaches at the lesser trochanter of the femur

Q: Is the iliopsoas a 2 joint muscle?

Technically the iliacus is a 2 joint muscle and the psoas is a 3 joint muscle, but often the SI (sacroiliac) joint is not counted, making the combined “iliopsoas” a commonly called 2 joint muscle.

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.

illustration of surrounding organs affected by tight hip flexor muscles

The iliacus and its intimate connection to the pelvic floor

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.

anatomy illustration of psoas and diaphragm
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 affects the strain of the nerves and muscles in the pelvic floor and hip regions, often manifesting in chronic tension and nerve pain.

anatomy illustration of hip flexors and pelvic floor muscles
The iliacus and its intimate connection to the pelvic floor

Summary FAQs:

Q: Does the psoas attach to the diaphragm?

The psoas and iliacus are connected to the diaphragm and the pelvic floor by the fascia connection.

Q: Does the iliacus and psoas attach to the organs?

The fascia in the body is continuous from head to toe and creates a connection between the iliopsoas and the digestive system, nerves, urinary system, lymph system, and reproductive system.

What do the iliacus and psoas muscles 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 muscle 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

illustration of twisted pelvis on full body alignment

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 muscle 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 vertebra 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, side bending, flexion and extension, but primarily, its job at the lumbar spine is to stabilize.

Why does my lower 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 excessive arch in their spine when they stand or lay down as well.

anatomy illustration of a tight iliopsoas causing lower back arching

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.

Summary FAQs:

Q: What is the function of the iliopsoas?

Primarily the iliacus and psoas are hip flexors but they also have different jobs due their location, nerve supply, and shape.

Q: What is the function of the iliacus?

The iliacus primarily stabilizes the hip and sacroiliac (SI) joint but when it does help with movement, it flexes and externally rotates the hip. It often causes an anterior rotation of the pelvis when the leg is stationary and can contribute to movements of the spine indirectly.

Q: What is the function of the psoas?

The psoas does most of its work stabilizing the spine (and to a lesser degree helps to stabilize the sacrum and hip). It also helps with hip flexion and hip external rotation as well as lumbar extension and other low back motions.

Q: Why does my leg rotate out when lying down?

The psoas and iliacus are hip external rotators and, when they are tight, they will make the leg rotate out to the side. When you release that tension and align the pelvis, the leg will not rotate out as much. The bony structure of the hip and pelvis can affect this too, but usually does so on both sides equally.

Q: Why does my back arch when I lay down?

A chronically tight psoas will pull the spine into extension (arch) at rest. A tight iliacus will rotate the pelvis forward, indirectly contributing to an arch in the back as well.

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

One of the most common causes of a pelvic rotation is a tight iliacus. Sometimes a tight psoas contributes as well.

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 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.

anatomy illustration of the muscles on the back side 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.

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.

Summary FAQs:

Q: What muscles oppose the iliopsoas?

The opposing muscles live on the back of the body and include



Gluteus Maximus

Gluteus Minimus

Part of Gluteus Medius

Obturator Internus

Q: Does the quadratus lumborum oppose the iliopsoas?

Quadratus lumborum does not produce hip extension and thus is not a direct antagonist to the iliopsoas but it does get tight along with the iliacus and psoas, both creating an anterior pelvic tilt and low back extension. Because of its position on the back of the body, quadratus lumborum often gets grouped as an antagonist muscle group to the hip flexors when it is not.

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 quadriceps, 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.

anatomy illustration of the hip flexor muscles

Summary FAQs:

Q: What muscles are synergists to the iliopsoas?

The muscles that work with the iliopsoas and also produce hip flexion include

Rectus Femoris


Tensor Fasciae Latae



Part of Gluteus Medius

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.