It’s no wonder the psoas are so misunderstood. The very process of naming these muscles connecting the top half of the body to the lower half has been a series of errors spanning four centuries.

Long before Hippocrates began using the modern Latin term psoa—muscle of the loins—ancient Greek anatomists called these muscles the “womb of the kidneys,” owing to their physical relationship to those organs. In the 1600s, the French anatomist Riolanus committed a grammatical error that survives to this day, identifying the two psoa muscles as the psoas instead of using the proper Latin psoai (Diab 1999)—a goof that would, in the end, perhaps influence our tendency to think of these muscles as a co-functioning team rather than as individual muscles that can adapt to our asymmetrical habits.

Adding insult to injury, in the 1950s John Basmajian, MD, 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. All this historical context makes it much easier to understand why there is widespread confusion about the actual role of the psoas.

Anatomical Review

When speaking of the psoas, it’s noteworthy to mention that while this term represents two muscles—the psoas major and minor—the latter is present in only about half the population. This article will focus specifically on the psoas major.

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 flex the hip? Does it move the spine? 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). Remember those grade school (old school!)–style sit-ups from the National Fitness Testing program (now known as the President’s Challenge Program)? During motions like sit-ups (which, oddly enough, are still part of the protocol), the psoas simultaneously extends the upper vertebrae while flexing the lower vertebrae, creating a shearing motion on the spine (one vertebra sliding forward on the other) and exerting substantial compressive loads (Bogduk, Pearcy & Hadfield 1992)—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 vertebral stabilization (keeping the vertebrae 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, lets the hip extend 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.

Assessing the Psoas

Because the psoas can shear the vertebral column forward, it is very common to see a “rib thrust” in those with a shortened psoas. Assessing this from a standing position can be difficult to do, though, as many people compensate for their tight psoas by slightly flexing their hips and knees to put a bit of “slack in the (psoas) line.” To keep your assessment objective, try this supine technique:

Begin with your client sitting on the floor with legs extended. The quadriceps should be fully relaxed and the hamstrings resting on the ground. Have the client begin to recline, stopping when the hamstrings lift away from the ground. At this angle, bolster your client under the head and scapulae, leaving space for the ribs to lower to the floor. The height of the bolstering will depend on psoas tension.

Ideally, we should be able to lie “in skeletal neutral” while on the floor. A shortened psoas will lift the thigh bones and/or the lower ribs away from the floor. This assessment is also the corrective. When you have identified that the psoas is indeed elevating the rib cage, instruct the client to relax until the lower ribs meet the ground. As your client releases, progressive shifting in the height or position of the bolster may be necessary.

Psoas or Iliacus?

Supine leg raises are a fairly common exercise in therapeutic and training situations. Ideally, raises should strengthen the legs, but most people actually do this exercise with their psoas instead of with the muscles needed for knee stability. How can you tell? Many exercisers will lift the leg while simultaneously tilting the pelvis posteriorly. While this is still technically a “leg raise,” the movement’s hinge is the lumbar spine—not the hip—which increases the load on the lumbar disks, fails to improve stabilization of the knee and further shortens the psoas. Teach leg raises a better way:

Have the client begin supine, with one knee bent and the other fully extended on the ground. Before beginning the exercise, the pelvis should be placed in neutral (with anterior superior iliac spine, aka ASIS, and pubic symphysis in the same horizontal plane). Cue your client to lift the straight leg to the height of the opposite knee without moving the pelvis.

Balancing the Psoas: What to Increase and What to Decrease

Perhaps never before have people been more informed via their healthcare team that their low back, hip, groin and pelvic issues are stemming from the mysterious psoas. This is why it is essential that movement professionals understand the anatomical features and biomechanical action of the psoas and know how to modify exercise programming to lessen the risk of an issue developing in the future.

It is no surprise why exercisers—from fitness and sport enthusiasts to elite professionals—have so many issues with their psoas. Performing a quick kinematic assessment of the most preferred exercise modes, you will find the majority of them are driven by hip flexion. And if you calculate and track a client’s habitual joint angles throughout the day, the results will likely show large periods (think hours!) sitting—at work, in the car, on the couch. Coupling work-time sitting with exercise-time sitting, the psoas doesn’t stand a chance with all that hip flexion!

While it may seem logical to balance a heavy hip-flexing habit with equal amounts of extension, this is not the best exercise prescription, as it does little to reduce the inappropriate amount of tension in the psoas—it just overrides it temporarily. Instead of adding extension exercises, look to replace common hip flexion activities like treadmill and bicycle workouts with in-line skating or cross-country skiing, actions driven more by hip extension. Talk to clients about reducing sitting time by using a standing workstation, and try to eliminate sitting (bikes, weight machines and so forth) during workouts. Consider adding a few psoas stretches and lengthening exercises like yoga’s warrior or lunge poses, paying special attention to the rib thrust. Rib thrusting during these exercises reduces their effectiveness in lengthening the psoas. Cue clients to lower the bottom ribs until they line up over the pelvis, to keep their muscular attachments in check.