There is no iliopsoas

If you’re interested in reading more on ideas presented in the article below, I suggest reading Alignment Matters. If you’d like movement instruction via video, start with Science of Psoas.

I’m back from three weeks on the road and boy is my iliopsoas muscle killing me.
Just kidding. There is no illiopsoas muscle. So why did we start calling two different muscles, that happen to share an attachment, by one name?

I’ve explained it more thoroughly in Can This Psoas be Saved (click), but in short, back in the 1950s, John Basmajian, the father of electromyograph (EMG) science, argued that the iliacus and psoas muscles could not be expected to have unique functions because they share a common lower attachment site. His opinion triggered widespread use of the term iliopsoas, stripping each muscle of its individual identity and setting the precedent of misattributing iliacus EMG measures to the deeper and more-difficult-to-read psoas muscle.

This happened in the 1950s.

It is now 2014 and we know a lot more, about the psoas, the limitations of EMG, and biomechanics in general.

The problem with grouping stuff into simple categories (like calling both the ilacus and the psoas muscle “hip-flexors”) is that, when you go to create corrective protocol (like undoing hip-flexor tension) you could be barking, or stretching, up the wrong tree.

For example, the psoas muscle attaches to 22 (sometimes 24) different structures in your body, most of them vertebral and vertebral disc connections. The lowest two attachments are on the thigh bone. Your psoas is psoa much more a spinal mover (or stabilizer in some cases) than it is hip mover. And a lot of the time (especially in chronic sitters) two tight psoa extend (and shear forward) the lowest thoracic vertebrae and the rib cage while simultaneously flexing the lower lumbar spine while tucking the pelvis under via the thigh bone moving forward.

Defining the psoas as a “hip flexor” means that this muscle swings the thigh bone forward about the axis of the hip  or anteriorly rotates the pelvis about the axis of the hip.


But the psoas (unlike actual hip flexors) doesn’t tilt the pelvis anteriorly, it tilts the pelvis posteriorly relative to the ground.

This is why the psoas should not be put into the hip flexor category. Or maybe the hip flexor category needs to be officially defined as muscles that move the thigh bone forward relative to the ground, but not necessarily relative to the pelvis. But technically — and we do want to be technical here, yes? — hip flexion is the motion of the thigh relative to the pelvis (which makes up half the hip joint), not the thigh relative to the ground.

Relativity* is everything.

To further complicate things, your tight hip flexors, like your iliacus and rectus femoris, could be tilting your pelvis anteriorly relative to your thigh


but at the very same time, your psoas could be post-tilting your pelvis (relative to the ground) while keeping the same degree of hip flexion as created by the hip-flexors.


In which case your pelvis might measure “neutral” relative to the ground, but your femur fixed out in front of you. Which means that the primary reason “neutral pelvis” is desirable — “neutral” maximizing the weighted, push-back phase of walking  — isn’t available.


“Neutral pelvis,” separated from other neutral parts, and separated from movement capacity, isn’t worth a damn. Alignment is not about position. It’s about your body’s ranges of motion and how those ranges of motion line up with the gravitational force. If your hip can’t extend because of hip flexors’ tension, your glutes and lateral hips are physically prevented from working while you walk. And P.S. Your pelvic floor misses its restoration team.

So. The moral of today’s story is, calling two muscles that do entirely different things by the same name makes it confusing when trying to figure out what’s going on in your body.

The other moral of today’s story is, calling the psoas a hip flexor is incorrect (and confusing) because the psoas not only does the opposite of what hip flexors do, it can also distort the vertebrae and move the rib cage wonky. And why would we want to put a muscle that can distort the spine and the upper part of the body into the HIP category where no one can find it when their back is hurting?

The other moral of today’s story is our anatomical reference system has become so sloppily used that we often refer to motions that are relative to the ground with terms that mean “relative to the joint.” Which is confusing. And incorrect.

The other moral of today’s story is, I just met my first “fan” out in the whole-wide world. There is nothing stranger than having someone say “Are you Katy Bowman?” while boarding a plane. I’ll never forget you, Dawn!


This was a milestone day and it gives me strength to keep on creating content because I know that you, (and you and you and you!) are finding it valuable.

“There is no such thing as an iliopsoas muscle.” – Stuart McGill.


The end.

*If you don’t have a strong grasp on the difference between the concepts of relative and absolute, this is where you might begin.

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19 thoughts on “There is no iliopsoas

  1. Thank you for this explanation. I have an anteriorly rotated pelvis and have been working on my quads and doing the lunge and another stretch to try and hit the illiacus. This post really helps to explain what’s going on. Thanks!

  2. Leah, try using a small to medium not-fully-inflated ball to get after the iliacus. Meaning: place the ball under the sacrum, knees bent, neutral pelvis or slight post tilt, and relax for a bit. Then, move the ball to the left and right, relaxing for a bit on each side, knees bent still. Then, let the ball leg (i.e., the side on which you have placed the ball) straighten out and get heavy; keep the other knee bent for stability. Make sure to position the ball in such a way that your backside ribcage is firmly planted on the ground and your pelvis neutral or post tilted. I’ve found this to be about the ONLY way to get a deep, iliacus release. Good luck.

    1. Thank you for this suggestion. I have been doing something similar using a bolster and will give this a try too.

  3. You are changing the life of each member of my family. Please don’t ever stop! You are beyond valuable.

  4. I always thought the psoas major muscle is the only ‘hip flexor’ that works past 90 degrees of hip flexion, and it has so many fibres that some can influence the pelvis posteriorly and some anteriorly depending on which are tight. And then what happens when it is weak and long??! Thanks for explaining a very hard to grasp concept.

  5. Thank you for speaking a language I can understand (well, not always, but I am getting there slowly). I’ll post this to my Pilates peeps – teachers and clients.

  6. So, this is me. I know this is the situation I am in but none of the professionals I’m in contact with seem to be aware of this (meaning weightlifting trainers and an oesteopath). Would you be able to tell me Katy please: what is the most important step for me to take next? I already stretch hamstrings and calves, I squat, I psoas release and I’m trying to increase walking. However, as I see it from this post, walking is not going to be beneficial if my biomechanics are so far out of whack, am
    I reading this right?

    Many thanks

  7. One of my students gave me link to your blogg. Good experience for a Tutor:-)
    So when I get started with my own blogg can I invite you to do a guest-blogg??
    Love bloggs with content and to get the discussions started:-)
    Have a lovely week.

  8. Your description of the psoas reflects my experience, Katy. Every once in a while, my psoas goes into spasm and I can feel my whole sacrum and hip being pulled. Next, my lower back and ribs seize up with pain. It is excruciating. It seems to be triggered by fresh blueberries or bananas as well as the standing position. The pain can be relieved if I lie down and hug my knees to my chest while deep breathing, then doing a variety of lengthening moves on the bed. What do you think is happening here?

  9. “If your hip can’t extend because of hip flexors’ tension, your glutes and lateral hips are physically prevented from working while you walk. And P.S. Your pelvic floor misses its restoration team.”

    That is very interesting. My glutes and lateral hips definitely aren’t working and my pelvic floor is not functioning correctly as I have pevlic organ prolapse. But what do we do about it? Your description is FANTASTIC and helps me to understand what’s NOT working. But I don’t know how to fix it. 🙁 I stretch my hamstrings, calf muscles, working on squat, all of the things that I can figure out from the book (not finished reading yet) but I haven’t seen anything on hip flexor stretching. Could you add a post script to point to the right place to find a stretch or an exercise for this? Thanks!!

  10. Thanks tremendously Katy, each time I reread your post it gives me another piece of understanding. To go off at a tangent, it also brings to mind an interview Tom Myers did with Liz Koch where he said he felt the human psoas acts in a different way to a four legged animal’s psoas, because we’ve straightened up onto two legs so our hips are extended an awful lot more in that position than in an animal that stands on four legs. (hope I’ve paraphrased him correctly there.) It made me wonder whether in animals with relatively inflexible backs (cow as opposed to cheetah) the psoas might act more as a hip flexor?

    Bit of a comparative anatomy nerd here!

  11. Katy! Thanks sooooo much for this post. It resonates with me on a level only an expert of your caliber could comprehend. Question: much if what your describing seems to be happening with me on my right side and now beginning on my left. I actually have physical pain, as if I had strained my hip, in certain position such as when I try to flex my R hip higher than 90 degrees. I’ve been doing some PNF stretches which seem to help, but I want to remedy the whole situation completely. Is the solution simply to stretch my iliacus and rectuanfekoris femoris? I would so love to see a follow-up post that gives some suggestions on what a person who’s going through this can do to fix the problem. Thanks! You’re the best!!!

  12. Hey just wanted to give you a quick heads up.
    The words in your ppost seem to be running off the screen in Internet explorer.
    I’m not sure if this iss a formatting issue or something to do with wweb browser compatribility but I figured I’d post to
    let you know. The design look great though! Hope you get
    the issue resolved soon. Many thanks

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