The Psoas, Reconsidered

Few muscles carry as much mythology as the psoas. It has been called the muscle of the soul, the seat of fear, the place the body stores its grief. If you have spent any time in somatic or yoga circles, you have heard at least some of this. You may also have wondered, quietly, how much of it is true.

The honest answer is that some of it is grounded, some of it is metaphor, and the two have become tangled. So let us untangle them, starting with the anatomy, because the real psoas turns out to be quite remarkable enough on its own.

Where the psoas actually lives

The psoas major is the deepest of the hip flexors. It begins high, at the sides of the bodies and transverse processes of the lumbar vertebrae, from roughly the last thoracic vertebra down through the lumbar spine. From there it travels down and forward, joins the iliacus, passes beneath the inguinal ligament, and attaches to the lesser trochanter of the femur. In anatomical terms it is the only muscle that links the lumbar spine directly to the leg (StatPearls, Iliopsoas).

This geography matters. Because it spans the spine and the femur, the psoas has a hand in hip flexion, in lumbar stability, and in how the pelvis and low back relate to one another. It is not a surface muscle you can grip. It sits behind the abdominal organs, close to the spine, which is part of why it has been so easy to mythologise. We cannot see it or touch it directly, so we imagine it.

The fascial neighbourhood

The psoas does not work in isolation, and this is where the more recent and more careful science becomes interesting. The muscle is wrapped in its own fascial sheath, and that sheath is continuous with the connective tissue of its neighbours.

Above it sits the diaphragm, the dome of muscle that drives breathing. The diaphragm has two tendinous arches at its back edge, the medial and lateral arcuate ligaments. The medial arcuate ligament passes directly over the upper psoas, and the fascia of the two structures is continuous there. Bruno Bordoni and Emiliano Zanier, in their 2013 review in the Journal of Multidisciplinary Healthcare, describe how the diaphragm's posterior fascia blends with the fascia of the psoas and the quadratus lumborum, knitting the breathing muscle into the deep architecture of the trunk (Bordoni and Zanier, 2013).

Follow that connective tissue downward and you arrive at the pelvic floor. The work of Carla Stecco and colleagues, set out in her Functional Atlas of the Human Fascial System (2015), has documented in cadaver dissection how the deep fasciae form continuous planes rather than separate envelopes. Fascia is not packaging. It is a connecting tissue, and through it the diaphragm, the psoas, and the pelvic floor share a structural conversation.

The diaphragm above, the psoas behind, the pelvic floor below. Three structures, one continuous tissue, all moving with the breath.

This is the physiological grain of truth beneath the poetry. When you breathe fully, the diaphragm descends and the pelvic floor responds, and the psoas lies in the middle of that movement. A held breath, a braced belly, a chronically gripped low back: these are not separate problems. They are one tissue under tension.

A densely wired muscle

There is a second reason the psoas behaves like more than a simple hip flexor. The lumbar plexus, the bundle of nerves serving much of the front of the leg, is embedded within the muscle itself. The psoas is innervated by the anterior rami of the upper lumbar nerves, roughly L1 to L3, and its branches pass through and emerge from the muscle belly (StatPearls, Iliopsoas).

It also sits in close company with the autonomic nervous system. The structures that regulate our unconscious state, including the sympathetic chain that governs the fight or flight response, run alongside the lumbar spine in the same deep territory. This proximity is anatomical fact. What it means functionally is where we should slow down and be careful.

What is metaphor, and what is plausible

The popular framing of the psoas owes a great deal to the somatic educator Liz Koch, whose The Psoas Book and later Stalking Wild Psoas (2019) describe it as an organ of perception, responsive to emotion and breath. Read as somatic philosophy, this is a thoughtful and useful lens. Read as anatomy, the phrase muscle of the soul is metaphor, and Koch herself writes in that register rather than claiming laboratory proof.

So what can we say with more confidence? A few things have real physiological grounding, without needing to overclaim.

• Chronic stress raises baseline muscle tone. A nervous system held in a defensive state tends to keep deep postural muscles, the psoas among them, guarded and shortened.
• The autonomic nervous system shapes how we hold ourselves. Stephen Porges's polyvagal theory, first set out in 1995, offers one influential model of how states of threat and safety map onto the body. It is a theory, still debated, not settled fact, and worth citing as such (Porges, 1995 and after).
• We genuinely sense our inner state. Interoception, the brain's reading of the body's physiological condition, is well established in neuroscience, notably in the work of A. D. Craig (Craig, 2002). A deep, breath-linked muscle is plausibly part of what we feel when we feel settled or braced.

Notice what this does and does not say. It does not say the psoas stores a memory of a specific event, or that releasing it discharges trapped emotion. There is no good evidence for that, and we should not pretend otherwise. What the evidence supports is gentler and, to my mind, more interesting: that a muscle this deep, this involved in breathing, and this close to the body's regulatory wiring will reflect our nervous-system state, and may in turn influence how that state feels to us.

Why slow, precise work matters

If the psoas were just a hip flexor, you could stretch it briskly and move on. Because it is woven into the breath and the deep nervous system, it asks for a different quality of attention. Fast, forceful effort tends to recruit the very guarding you are trying to ease. Slow, patient, breath-led work gives the tissue and the nervous system time to register that there is no threat, and to let go.

That is the case for precision over drama. Not the muscle of the soul, but something quieter and truer: a deep, intelligent, well-connected muscle that responds best when we approach it with the same calm we are hoping to find.

Key takeaways

• The psoas is the deepest hip flexor and the one muscle linking the lumbar spine to the femur, sharing continuous fascia with the diaphragm and pelvic floor.
• Its dense innervation and closeness to autonomic structures make it genuinely responsive to nervous-system state, while claims that it stores specific emotions remain metaphor, not evidence.
• Because it is bound up with breathing and regulation, slow and precise work serves it far better than force.

Sources

1. Siccardi MA, Tariq MA, Valle C. Anatomy, Bony Pelvis and Lower Limb, Iliopsoas Muscle. StatPearls, NCBI Bookshelf, updated 2023. https://www.ncbi.nlm.nih.gov/books/NBK531508/
2. Bordoni B, Zanier E. Anatomic connections of the diaphragm: influence of respiration on the body system. Journal of Multidisciplinary Healthcare, 2013;6:281-291. https://pmc.ncbi.nlm.nih.gov/articles/PMC3731110/
3. Stecco C. Functional Atlas of the Human Fascial System. Churchill Livingstone / Elsevier, 2015. https://www.sciencedirect.com/book/9780702044304/functional-atlas-of-the-human-fascial-system
4. Porges SW. Orienting in a defensive world: mammalian modifications of our evolutionary heritage. A polyvagal theory. Psychophysiology, 1995;32(4):301-318; and Porges SW. The polyvagal theory. Cleveland Clinic Journal of Medicine, 2009. https://pubmed.ncbi.nlm.nih.gov/19376991/
5. Craig AD. How do you feel? Interoception: the sense of the physiological condition of the body. Nature Reviews Neuroscience, 2002;3(8):655-666. https://www.nature.com/articles/nrn894
6. Koch L. Stalking Wild Psoas: Embodying Your Core Intelligence. North Atlantic Books, 2019; and Koch L. The Psoas Book. Guinea Pig Publications. https://www.northatlanticbooks.com/author/liz-koch/