A response to Katy Bowman’s ‘There is no iliopsoas’

To kick things off, I want to tackle a question posed by a friend on Facebook, and in so doing, I pray that I do not start a war with the venerable Katy Bowman, but maybe we can help each other learn more, and then go for waffles.

My friend read a blog on katysays.com, in which Katy says there is no such thing as the ‘iliopsoas’ muscle. My friend asked a bunch of us to say what we thought about that. At first I just reacted to the title of the article, and the evidence of that is in PART I of my response. But then I read a paragraph from the article, and several articles on the iliopsoas  that made me want to say a little more in PART II. I provide links to the abstracts of these articles or the articles themselves. Unfortunately, unless you have an institutional library access, you might only get to read the abstract from the link, so although I apologize for that, I won’t apologize for using peer-reviewed sources only on this site. I encourage you to read any full article, including Katy’s, rather than take my word for it.


In Katy’s insistence that there is no iliopsoas muscle, she is being at little pedantic, but for me to say that is certainly the pot calling kettle black, so please, let’s get that on the table right away. Yes, of course there is no iliopsoas muscle, but in reality, the sun doesn’t ‘rise’ –  the earth rotates. Still we say ‘iliopsoas’ and ‘sunrise’. Some of it is quaint convention, and some of it has to do with perception, and other parts have to do with the way things really are. I will leave sunrises for poets and cosmologists, and I will talk about muscles.

We have many muscles with special names because they share a common tendon. The quadriceps for example describes four muscles with a common tendon at the knee. The three vastii join to the femur and the rectus femoris crosses the hip. Only one of them flexes the hip, one of them pulls the kneecap laterally, one of them pulls the kneecap medially, but they all extend the knee. The pectoralis major is more radical, with sternal and clavicular divisions that can make the muscle act as its own antagonist in flexion and extension. There is also the triceps brachii, the triceps surae, biceps brachii, biceps femoris among others. Individual muscles with distinct and different functions, grouped together by a common tendon on one end.

In the case of the iliacus and psoas muscles, the iliacus inserts proximally on the upper part of the inside of the iliac bone, and the psoas major (don’t get started on the psoas minor, which is only present in 56% of the population, differs by race, side of body and sex…) has a proximal insertion on the 12th rib and the transverse processes of T12-L5 (ish). The two muscles co-join at the tendon which inserts onto the lesser trochanter of the femur, although some fibres of the iliacus continue further down the femur. In actual fact, most of the fibres of the iliacus insert not onto the lesser trochanter itself, but onto the tendon of the psoas. A clear case of riding in on another muscle’s coat-tails!

The fascia of the psoas muscle is continuous- and joins with- the fascia of the iliacus muscles as soon as it descends below the lip of the iliac crest. Psoas major and iliacus are both innervated by the second and third lumbar nerve, which I think is very important for our discussion here.

When acting together, the two muscles flex the hip, and may cause some adduction, and some inward or outward rotation of the femur, depending on the angle of the femur at the beginning of the movement. In terms of flexion (the principal action) they lift the femur if the pelvis is fixed, and cause one to sit up if the femur is fixed. Although EMG studies are difficult on these muscles since you need painful needle electrodes, and lots of money to compensate your volunteers, we do know that in hip flexion they always act together, and of course this makes total sense: any synergy of your body would not allow for a perfectly situated muscle to just ‘go along for the ride’. If it is there, and available, it will contribute. This is because they are innervated by the same nerves! The motor cortex makes a plan to move the leg, and uses all the possible muscles for that particular movement, it sends the command down the appropriate branch of the nerve, and the motor units are recruited, and voila! The leg moves.

I have seen no literature, either by pathology or by nerve block where the iliacus acts independently of the psoas, but it might be out there. On the other hand, the psoas does many things without the iliacus. The psoas stabilizes the pelvis in locomotion, can flex the lumbar spine if the thorax is in static flexion above T12, can hyperextend the lumbar spine if the thorax is allowed to extend or hyperextend above T12, can cause lateral flexion, and other complex movements. Its action is sometimes considered paradoxical or enigmatic, and its role overall is poorly understood given its apparent importance.

Are they separate muscles? Well, sure. But so are long and short heads of the biceps brachii. If Katy is going to apply this rule to the iliopsoas, she needs to apply it to all other multi-head muscles as well. And she should also stop saying ‘sunrise’.

I would also add that Katy doesn’t like calling the psoas a hip flexor because it does other things. That is not a just reason to make such a claim. A muscle can (and must) have than one function, otherwise you would need a bajillion muscles. As it is, a muscle’s function is contextually derived, and depends on what synergist and neutralizer muscles are available. Saying the psoas doesn’t flex the hip because it has other jobs is like saying the biceps doesn’t supinate the forearm because it flexes the elbow, or like saying Brian May isn’t a PhD in astrophysics because he was the guitarist for Queen. Well he is both. Silly rabbit, the psoas can do both.


Okay, I have now read Katy’s article more thoroughly, as well as a few research articles about the psoas, and I take back what I said earlier about her article: There are parts of it I dislike.

What I dislike in particular is the sentence that begins “But the psoas (unlike actual hip flexors) doesn’t tilt the pelvis anteriorly, it tilts the pelvis posteriorly relative to the ground.” There are a few problems here, not the least of which is her unclear nomenclature. When the ASIS moves forward relative to the pubic bone it is typically called anteversion. When the ASIS moves backward relative to the pubic bone, it is called retroversion. I believe she is saying the psoas does not antevert the pelvis, but it retroverts it.

We get into interesting territory now. Let’s go back to my earlier example of the pec major. The pec major has one common attachment on the humerus, just under the deltoid, but proximally, it has one attachment on the sternum and one on the clavicle. I could say: pec major is a horizontal flexor of the shoulder, and nothing else! Of course, the pec major (clavicular) is also a shoulder flexor, while pec major (sternal) is a shoulder extensor. I could stop there and say, well, isn’t that odd? Pec major does three different things! But let’s go on! If I start with my shoulder hyperextended (that is behind anatomical neutral), then pec major sternal (remember, the shoulder extensor?) now becomes a shoulder flexor. We could claim that the muscle only has one role, and that anyone who believes otherwise is wrong wrong wrong. Or we could accept that the initial conditions of the system determine how the muscle will function.

In simple point of fact, the only thing a muscle really does is get shorter. That is how it works. If a muscle has one goal in life, it is to bring its two insertion points closer together. With muscles that cross only one joint, this is pretty simple. Brachiallis at the elbow gets shorter and the elbow flexes. Biceps brachii, on the other hand, crosses the elbow and the shoulder, as well as spanning the space between radius and ulna. So when the two ends try to get closer together, there are many possibilities: The shoulder could flex; the elbow could flex; the radio-ulnar joint could supinate. To accentuate the desired effects, the brain fires synergists. So if elbow flexion is desired, brachiallis also fires. However, if shoulder flexion and supination are NOT desired, the muscular synergy must include a shoulder extensor to neutralize the shoulder flexion, and a pronator to neutralize the supination. The biceps is relatively simple compared to the psoas, but you can already see the layers of complexity in this simple muscle. To understand the many functions of the psoas, imagine 5 thread spools, connected with a string end to end through the middle, and these are attached with a hinge to the side of a sink, tilted on its side. Attached with a hinge to the bottom side of the sink is a stick. Now imagine you have a string connecting the top spool to the stick. If you tightened the string, would the stick move forward? would the spools? would the sink? Maybe the spools would collapse on one side while the sink moves backward and the stick stays still? Of course, the correct answer is ‘It depends’. If you could stabilize the spools in some way, or neutralize the movement of the stick, or if the spools started in front of the sink… all of these things and more contribute to the many potential functions of the psoas. The ‘furthermore’ in all of this is that people who produce more force in hip flexion have greater cross-sectional area of the psoas (Kubo et al, 2010Copaver et al. 2012), hip flexion has been shown to be weakened after LIF surgery, which interferes with psoas but not iliacus function (Lee et al. 2013) and the psoas has been shown to contribute to hip flexion (Lewis et al. 2009) and both hip and lumbar flexion, as well as some other complex movements (Andersson et al. 2007). By contrast, Hu and others (2013) found that when people performed straight leg raises while standing on one leg, the psoas was active on both standing and gesture sides. They interpreted this to mean that maybe the psoas was only acting to stabilize the spine. Of course, maybe the muscle was stabilizing the spine on the standing side, but lifting the leg on the gesture side. On the balance of the evidence, I maintain that the psoas has many and varied roles, but to claim that it is not an ‘actual’ hip flexor is a rejection of the ‘actual’ scientific evidence. And thus endeth the rant. As well as my pedantry.

I don’t dislike her article, and I love Stu McGill, whom she quotes, but there are babies and there is bath water, and we have to make some distinctions…

Don’t get me wrong. Having looked over the website and Katy Bowman’s excellent credentials and thinking, I hope Katy has the opportunity to influence lots of thinking around anatomy and biomechanics, because she is clear, down to earth, and cares about stuff I also think is important. I just have a few quibbles with some things written in her post. I hope to add a couple of video examples to make an illustration, but that will have to wait.

Feel free to comment, if I can figure out how to turn that on…