Do You Know Psoas?

If you know nothing about anatomy, chances are you have never heard of the psoas muscle and struggle with the pronunciation (hint: “so-azz”). If you associate with the new wave of so called movement enthusiasts or you are even remotely into fitness, the psoas is probably your favourite muscle. I am not kidding — when my thirst for knowledge related to all things yoga and movement graduated from a keen interest into a mild obsession, I realized just how revered this muscle is. For one, David Keil, the creator of the Yoganatomy website and author of my favourite yoga anatomy book, The Functional Anatomy of Yoga has hailed the psoas as “almighty”1 in this blog post. In fact, he has an entire psoas resource page here. For two, almost every yoga blogger has a post about the psoas — so here is mine.

I am not here to challenge the almighty status of anyone’s cherished core muscle — but I think there is a paradox at play that should be discussed. That is, the psoas is actually a lot less understood than some movement enthusiasts would have you believe. When we begin digging into the scientific literature, we see that our beloved psoas is actually quite mysterious and misunderstood. But please do not take offence — I believe the research demonstrating how mysterious this muscle is only reinforces psoas’ almighty status.


What we do know — A powerful hip flexor

Before I delve into the mysteries, let’s quickly review what we know. The psoas major muscle originates around the upper lumber or thoracic vertebrae and inserts on the lesser trochanter of the femur (i.e., the top of the thighbone; see Figure 1). The muscle has bundles of fibers — or “fascicles” — that attach all the way down the vertebral column. These origin/insertion sites give the psoas its unique ability to act on both the hip joint and the lumbar portion of the vertebral column as a powerful hip flexor2. And while nobody disagrees with this conclusion, researchers believe that hip flexion may be a narrow way to see psoas. Specifically, it has been suggested that the precise role of the muscle may be dependent on the flexion angle at the hip joint, which I will delve into in a moment 2,3. Confusion surrounding psoas is not limited to its function — the literature is rife with contradiction in many of the corners of the psoas related literature, some of which I will explore today.

20171120_163213Figure 1. The psoas major muscle.

Psoas Variation

There is much variation across individuals regarding the psoas muscle. For example, in 15% to 40% of individuals, psoas major is accompanied by a second smaller muscle, psoas minor4 — this is a strikingly large range! Further, the muscle originates on either the upper lumbar (i.e., L1) or the lower thoracic (i.e., T12) vertebrae, however, the prevalence of each origination is unknown5. And while these details may seem inconsequential, the biomechanical implications of these anatomical variations has not been closely examined 5.


Psoas Function

While some equate the psoas as being the most powerful hip flexor, new research is demonstrating that hip flexion may not be its primary purpose. In fact, one study using an experimental model to assess psoas muscle activity during hip flexion found that the psoas doesn’t even act as a hip flexor until the hip is already flexed to 45° (presumably as a result of other hip flexors, such as the quadriceps muscle rectus femoris)2. Prior to its hip flexing action (i.e., during the first 0-45° of hip flexion), Yoshio et al. (2002) proposed that the psoas works mainly to stabilize the femur and the lumbar spine2 (see Figure 2 (A)). Another study found that psoas, unlike rectus femoris, adductor longus, and iliacus (the sister muscle group of the psoas, think “iliopsoas complex”), was contralaterally active during an active straight leg raise (see Figure 2 (B))6. In plain language, if you lie down on your back and raise your right leg (i.e., flex your right femur), the powerful muscles in your left leg will stay silent — but psoas will keep firing. The authors attribute this finding to suggest that psoas is not acting simply as a hip flexor but is likely being recruited to stabilize the lumbar spine6. These are just two studies, but it is evident that even the basic function of psoas is still being uncovered.

Psoas 2Figure 2. (A) Function of the psoas muscle at different degrees of hip flexion based on the experimental model conducted by Yoshio et al., (2002). (B) Muscle activity in four muscles during an active straight leg raise in the ipsilateral (i.e., flexing) and contralateral (i.e., non-flexing) legs based on the work conducted by Hu et al., (2011).  


Psoas and Low Back Pain

The literature is unclear about what role the psoas plays in patients with chronic low back pain. A recent systematic review (i.e., paper that summarizes all the papers) looked at studies that assessed the cross-sectional area (CSA; a marker for muscle atrophy) of various trunk muscles in patients with chronic low back pain7. They found conflicting evidence to support a relationship between changes in CSA of psoas and patients with low back pain (unlike the multifidus muscle CSA which they found to be robustly predictive of low back pain)7. Two of the studies included in this review exemplify the conflicting evidence. While a 1993 study found psoas CSA in low back pain patients to be smaller than that of their healthy controls, a similar study published in 2000 found no changes in psoas CSA 8,9. Thus, unlike the multifidus muscle to which there is a clear relationship with the muscle’s atrophy and presence of low back pain, it is unclear how psoas muscle atrophy relates to chronic low back pain.


Why we might care

So why is all of this important and how does it relate to yoga? I promise: it does. A Google search of “psoas”, “low back pain”, and “yoga” yields 477,000 results rich in advice on how to “release” your psoas to relieve your low back pain. And while there is evidence supporting the use of yoga as an intervention in the treatment of low back pain10, the evidence is not as robust as our initial Google search would have us believe. A recently published systematic review examining the effects of yoga for improving symptoms of chronic low back pain found that, after one year of intervention, yoga was only slightly better than individuals receiving education alone (i.e., non-exercise controls) but bore no benefit over other forms of exercise (i.e., exercise controls) 10. Now before you throw away your mats, I don’t necessarily see this as completely discouraging. In light of the discussion today, here is what I believe is the take home message: all things psoas are still being uncovered in the research — from its very function, to its biomechanical properties, and its possible role or contribution in low back pain. Thus, when I read a study that shows how yoga may only be marginally good at improving low back pain, its no wonder when we barely understand the mechanisms behind low back pain and the function of psoas to begin with! Imagine being asked to fix a problem (low back pain) with a tool (yoga) without really understanding the underlying mechanisms at play? If we can inform our yoga practice with the best evidence then we can continue to improve our ability to target these issues.



  1. Keil D. The Functional Anatomy of Yoga. Chichester, England: Lotus Publishing; 2014.
  2. Yoshio M, Murakami G, Sato T, Sato S, Noriyasu S. The function of the psoas major muscle: passive kinetics and morphological studies using donated cadavers. J Orthop Sci. 2002;7(2):199-207. doi:10.1007/s007760200034.
  3. Andersson E, Oddsson L, Grundström H, Thorstensson A. The role of the psoas and iliacus muscles for stability and movement of the lumbar spine, pelvis and hip. Scand J Med Sci Sports. 2007;5(1):10-16. doi:10.1111/j.1600-0838.1995.tb00004.x.
  4. Bogduk N, Pearcy M, Hadfield G. Anatomy and biomechanics of psoas major. Clin Biomech. 1992;7(2):109-119. doi:10.1016/0268-0033(92)90024-X.
  5. Faisal SB, Al Bastaki UMH, Pierre-Jerome C. The Psoas Major Muscle at the Lumbar Spine. Contemp Diagnostic Radiol. 2017;40(11):1-7. doi:10.1097/01.CDR.0000516480.38552.a2.
  6. Hu H, Meijer OG, van Dieën JH, et al. Is the psoas a hip flexor in the active straight leg raise? Eur Spine J. 2011;20(5):759-765. doi:10.1007/s00586-010-1508-5.
  7. Ranger TA, Cicuttini FM, Jensen TS, et al. Is the size and composition of the paraspinal muscles associated with low back pain? A systematic review. Spine J. 2017. doi:10.1016/j.spinee.2017.07.002.
  8. Parkkola R, Rytökoski U, Kormano M, Hides J. Magnetic resonance imaging of the discs and trunk muscles in patients with chronic low back pain and healthy control subjects. Spine (Phila Pa 1976). 1993;18(7):830-836. doi:10.1097/00007632-199612010-00011.
  9. Danneels LA, Vanderstraeten GG, Cambier DC, Witvrouw EE, De Cuyper HJ, Danneels L. CT imaging of trunk muscles in chronic low back pain patients and healthy control subjects. Eur Spine J. 2000;9(4):266-272. doi:10.1007/s005860000190.
  10. Whitehead A, Gould Fogerite S. Yoga Treatment for Chronic Non-Specific Low Back Pain (2017). Explor J Sci Heal. 2017;13(4):281-284. doi:10.1016/j.explore.2017.04.018.






3 thoughts on “Do You Know Psoas?

  1. Great post Kerry! After finally seeing an actual set of psoas in medicine this year, I have a greater appreciation for the muscle made famous in fitness and yoga circles. That said, and as highlighted in your post, for all their fame, much remains unknown or misunderstood about the psoas muscles.


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