Psoas Major: Origin, Insertion, Nerve, Action & The "Hidden" Back Pain Culprit

The Psoas Major (pronounced "SO-as") is a long, fusiform muscle located deep in the abdominal cavity, running along the lumbar spine. It is the only muscle that connects the spine directly to the legs. It joins the Iliacus to form the Iliopsoas, the body's most powerful hip flexor.

[Image of Psoas Major muscle anatomy]

Quick Anatomy Snapshot

Origin (Proximal)	Vertebral Bodies: Sides of T12-L4. Intervertebral Discs: Between T12-L4. Transverse Processes: L1-L5 vertebrae.
Insertion (Distal)	Lesser Trochanter of the Femur (via the Iliopsoas tendon).
Nerve Supply	Anterior Rami of Lumbar Plexus (L1, L2, L3) - Direct branches, not the Femoral nerve.
Blood Supply	Lumbar arteries, Iliolumbar artery.
Primary Actions	Hip Flexion: Lifts the thigh towards the trunk. Trunk Flexion: Bends the trunk forward (like a sit-up) if legs are fixed. Lateral Flexion: Bends trunk to the same side (unilateral).

Deep Dive: The "Filet Mignon"

In animals, the Psoas Major is known as the tenderloin (e.g., Filet Mignon). In humans, it is a critical structural bridge.

1. The Lumbar Plexus Bed

The Psoas Major has a unique relationship with the nervous system. The Lumbar Plexus (network of nerves including the Femoral and Obturator nerves) forms within the posterior substance of the Psoas muscle belly. Tightness or spasm in the Psoas can entrap these nerves, causing referred pain.

2. The Diaphragm Connection

The upper attachments of the Psoas Major merge with the crura of the Diaphragm via the Medial Arcuate Ligament. This fascial connection means that breathing patterns and stress can directly affect Psoas tension, and vice versa.

Physio Corner: Clinical Relevance

💪 Functional Fact: The Psoas is the primary engine of walking and running. It initiates the swing phase of gait. In sitting, it is constantly shortened. Chronic sitting leads to "adaptive shortening," which pulls the lumbar spine into hyperlordosis (swayback) when you stand up.

Palpation

Palpating the Psoas is invasive and uncomfortable because it lies deep to the abdominal organs (intestines).
Technique: Patient supine, knees bent. Locate the navel and ASIS. Place fingers halfway between them. Press slowly and deeply posteriorly toward the spine. Ask the patient to gently lift their foot (flex hip). You will feel the Psoas contract deep in the abdomen.

⚠️ Clinical Pathology: Psoas Syndrome
A tight Psoas can compress the lumbar discs and facet joints. It can also cause Internal Snapping Hip Syndrome, where the iliopsoas tendon snaps over the iliopectineal eminence or femoral head, creating an audible "clunk" in the groin during hip extension.

Manual Muscle Testing (MMT)

We test the Iliopsoas complex (Psoas + Iliacus) together.

Testing Tip: Testing hip flexion with the hip Externally Rotated and Abducted slightly can bias the Psoas line of pull.

Step-by-Step Procedure (Oxford Scale)

Grade	Patient Action & Resistance
Grade 3, 4, 5 (Against Gravity)	Position: Sitting (legs dangling). Action: Patient lifts the thigh off the table (flexes hip), keeping the knee bent. Resistance: Applied at the distal anterior thigh (just above the knee), pushing down. Grade 3: Lifts thigh clear of table. Grade 4/5: Holds against moderate/strong pressure.
Grade 2 (Gravity Eliminated)	Position: Side-lying on the non-test side. Therapist supports the test leg. Action: Patient flexes the hip (brings knee toward chest). Result: Full range of flexion.
Grade 0, 1 (Palpation)	Action: Palpate medial to the Sartorius origin, just distal to the inguinal ligament. Cue: "Try to bring your knee to your chest." Grade 1: Contraction felt in the femoral triangle. Grade 0: No activity.

Frequently Asked Questions

Does the Psoas extend the spine?

It depends on posture. In a normal spine, it acts as a vertical stabilizer. In a swayback (lordotic) spine, it pulls the vertebrae forward, increasing extension/lordosis. In a flat back, it can assist flexion.

What is the Psoas Minor?

The Psoas Minor is a small muscle that sits on top of the Psoas Major. It has a long tendon and acts to weakly flex the trunk. It is absent in about 40-50% of the population.

Why does Psoas tightness cause back pain?

Because it attaches to the lumbar vertebrae and discs, a tight Psoas exerts a constant anterior shear force and compressive load on the lumbar spine, leading to facet joint irritation and disc strain.

Test Your Knowledge: Psoas Major Quiz

1. What is the primary insertion of the Psoas Major?

2. Which nerve plexus forms within the Psoas Major?

3. The Psoas Major originates from which vertebrae?

4. Which nerve supplies the Psoas Major?

5. What is the Thomas Test used for?

6. The Iliopsoas is the primary muscle for:

7. Internal Snapping Hip Syndrome involves the Psoas tendon snapping over:

8. Which organ lies directly anterior to the Right Psoas Major?

9. The Psoas Major enters the thigh by passing under the:

10. True or False: The Psoas Major can laterally flex the spine.

References

• Moore, K. L., Dalley, A. F., & Agur, A. M. R. (2018). Clinically Oriented Anatomy. 8th ed. Philadelphia: Wolters Kluwer.
• Standring, S. (2016). Gray's Anatomy: The Anatomical Basis of Clinical Practice. 41st ed. Elsevier.
• Travell, J. G., & Simons, D. G. (1999). Myofascial Pain and Dysfunction: The Trigger Point Manual. 2nd ed. Williams & Wilkins.