The Mighty Psoas: What It Does, Why It Matters

While the psoas major muscle affects many fitness activities, there is widespread confusion about its actual role in the body. What does this muscle do, and why is it shortened in so many people?

Anatomy of the Psoas

While the psoas major may seem like one long muscle that passes over multiple joints, dissection reveals a slightly different story. The body of each psoas contains (on average) 11 branches of muscle fibers—fascicles with separate attachments to bony sites—with the most superior fascicle attaching to the lowest thoracic vertebra, while others continue on down to various sites on the lumbar spine and to a final attachment on the femur. In addition to attaching on the transverse processes of some vertebrae, the psoas attaches directly to each of the lumbar spine’s intervertebral disks. This adds up to a total of 22 attachments: one on each thigh and 20 points of attachment on the spine.

The psoas has two layers—superficial and deep. Embedded between these muscles is the lumbar plexus, a dense collection of nerves that innervate the transverse and oblique abdominals, the pelvic floor, the deep hip rotators and most thigh muscles (Kirchmair et al. 2008). Because the psoas attaches at multiple locations, passes over multiple joints and entraps a major neurological network, it is no wonder that so many injuries can be blamed on one misbehaving muscle.

Psoas Mechanics

In light of all those connection points, you might ask, Does the psoas move the spine? Does it flex the hip? Or does it do a little of both?

Biomechanists are always trying to figure out what a muscle is “supposed” to be doing, considering joint health, leverage and force production. All those connections to the spine seem to imply that the preferred role of the psoas would be to somehow move the spine. But tests of this hypothesis show that the angles of attachment don’t allow much force production during side bending (lateral flexion). During motions like sit-ups, the psoas simultaneously extends the upper vertebrae while flexing the lower vertebrae, creating a shearing motion on the spine—not the most preferred movement for long-term spinal health.

Studies show that the psoas has an active role in hip flexion, but compared with the iliacus, the psoas plays a larger role in stabilizing the vertebrae (keeping them from rotating in the frontal plane) than in generating leg motions (Hu et al. 2011). And finally, the many attachments make it extremely important that the psoas can lengthen enough to allow the spine, pelvis and hips to articulate and move naturally for a pain-free and injury-free body.

The “Sitting” Psoas

If you’ve ever watched a triathlete transition (decrepitly) from the cycling phase of a race to the running portion, you have a sense of how a long bout with a tightening psoas can affect our ability to walk upright. In a slightly less extreme way, hours (and hours and hours) in the sitting position affect the psoas’s ability to extend to its full length—a length that allows us to stand in alignment and, perhaps more crucially, allows for hip extension as we walk. If you quickly calculate how many of your clients move from 8 or more hours of sitting at their jobs to “fitness” activities that further perpetuate the skeletal arrangement of a shortened psoas (cycling, stair-stepping, sitting on weight machines), then it is no surprise that the exercising population has so many psoas-related issues of the low back, pelvis and hips.

What Does a Short Psoas “Look” Like?

It is not uncommon for professionals to spot an exaggerated curve in the “low back” and conclude that the client has an anterior pelvic tilt. This form of postural assessment is flawed, however, because there are no objective data points to determine actual skeletal position or where, specifically, the curve is occurring. The spinal curve from a tight psoas is not a hyperextension of the lumbar spine, nor does it result from an anterior pelvic tilt. It is, rather, a unique curve created by anterior displacement (shear) of superior vertebrae coupled with various degrees of extension and flexion in the upper and lower lumbar spine, respectively. It looks very similar to hyperlordosis, with the exception of one bony marker: the rib cage.

For a reference list, plus information on how to assess a shortened psoas in your clients and what exercises can lengthen the muscle, please see “Can This Psoas Be Saved?” in the online IDEA Library or in the October 2012 issue of IDEA Fitness Journal.

For the latest research, statistics, sample classes, and more, "Like" IDEA on Facebook here.

Katy Bowman, MS

IDEA Author/Presenter
January 2013

© 2013 by IDEA Health & Fitness Inc. All rights reserved. Reproduction without permission is strictly prohibited.

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Article Comments

Daniel Lovelace
On Jan 02, 2013
Great description and analysis of a little known or talked about muscle with vast implications to the fitness professional. So many of my clients suffer from some degree or another of lower back pain or weak abdominals that translate to core dysfunctions. This article will help me explain to them how this works and the importance of core training and stability.
Anonymous
On Jan 03, 2013
This is great. Shows me just how much I need to forget. I was under the impression the psoas "pulled" the pelvis anteriorly, creating hyperextension in the lumbar.

Thank you IDEA.
Anonymous
On Jan 03, 2013
People should not sit for long hours. They should stand.walk move around. Changes in some work places help by having standing as opposed to sitting work stations. Popele are too fat, too weak, too unfit and yet are surprised when they ahve back pain.
Isolating one muscle is not helpful. I think whole body posture and integrated movement should be taught to clients. Do functional multi joint exercise.
Too much isloation work is taught in gyms
Too many machines that require sitting and doing exercise
Alana Levin
On Jan 03, 2013
Great subject and the anatomy of the psoas was fascinating!
A photo would be helpful (of the shortened psoas on a person to see how they stood)
Thanks
Martin Haines
On Jan 04, 2013
Great article. Interestingly we have tested over 4,000 people from olympic athletes to the sedentary and have found that very few have 'tight' hip flexors. The problem is the standard Thomas test to measure hip flexor integrity is probably flawed and so if using this test to identify tight hip flexors people may be getting skewed data.
Anonymous
On Jan 04, 2013
Informative article, makes me want to read the archived article on saving the illiopsoas. The illiopsoas seems to be one of the terrorists of back pain. Looking forward to reading more of your articles.
Deanne DeForest
On Jan 04, 2013
Very interesting article. As visual person, when I look at the anatomical shapes and attachments of muscles on bone (and the tendon/ligament structure), I visualize the type of movement that can potentially occur. Add organs, and other muscle layers (and those actions), and there is yet another picture of function.

When I saw this article, and reviewed the attachment sites, the location of psoas inside the pelvic girdle, my first thought was that this was a pelvic and spinal stabilizer, offering deep proximal support to the impacts of upper body weight and movement while upright.

To place this into a stretch, I might consider rolling a client into supported spinal extension on a large pilates ball, while abducting and extending the hips. The abds, obliques would receive a good stretch in this position too.
Stacie Pine
On Jan 06, 2013
Yes, a picture would be very helpful.
Paul Brown
On Jan 06, 2013
Thanks Katy for a very simple and practical explanation of the Psaos and thanks especially for not using the word "core" once throughout your entire article, possibly otherwise the second most overused four letter word on the internet. Concurring with Deanne, the key implication is the start of every workout must be a progressive full body warm up mostly in the UPRIGHT position. Not only does this open up and allow for greater blood flow to the muscles it also releases any unnatural postural restrictions and allows our body to find its true center of gravity in preparedness for movement in all directions. The Psaos is just one of a number of muscles unhappy to be constrained in range by being tied to a chair or car seat for hours, so like your triathlete analogy, let's transition our fitness clients into their workouts in a moderate, upright and progressively multi directional activity.
Charles www.pthub.com
On Jul 25, 2013
Very interesting article. Check out the work of Liz Koch on the Psoas too.. Its a fascinating muscle that is also affected by the fight or flight response system, dramatically shortening at times of high stress / fear
Barry Edwards
On Jan 06, 2014
Yes the Psoas can cause many problems and when reviewing its anatomy it is easy to see why Sit ups , crunches etc are harmful to the back and do not constitute what too many people view as "core" exercises. A picture would indeed be helpful as it would show the extreme angle that psoas crosses the pelvis at as viewed in standard anatomy texts it appears as though the muscle runs from the spine to the femur in a straight line which is not the case.
Magdalena Olesiak
On Jan 16, 2014
I so agree about the huge role of PSOAS in our optimal health&fitness performance!

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