Iliocostalis Lumborum: Origin, Insertion, Action & Low Back Pain

The Iliocostalis Lumborum is the lowest and thickest part of the Iliocostalis muscle, forming the lateral column of the Erector Spinae in the lower back. It is a primary generator of extension force for lifting objects and is a frequent source of chronic lower back pain.

[Image of Iliocostalis Lumborum anatomy]

Quick Anatomy Snapshot

Group	Erector Spinae (Lateral Column).
Origin (Proximal)	Common Extensor Tendon: Iliac Crest (lateral lip), Posterior Sacrum, and Spinous Processes of T11-L5 and the Supraspinous Ligament.
Insertion (Distal)	Inferior borders of the angles of the Lower 6 or 7 Ribs (Ribs 6-12).
Nerve Supply	Lateral branches of the Posterior Rami of thoracic and lumbar spinal nerves.
Primary Actions	Bilateral: Extension of the spine (straightening the back). Unilateral: Lateral flexion (side bending) to the same side. Stabilization: Resists anterior shear forces on the lumbar spine.

Deep Dive: The Lifter's Muscle

The Erector Spinae group (Iliocostalis, Longissimus, Spinalis) is the powerhouse of the back. The Iliocostalis Lumborum is the "anchor" of this group.

1. The Common Tendon

In the lower back, the erector spinae muscles are not distinct fleshy bellies but are fused into a broad, thick, white sheet called the Thoracolumbar Fascia (or the common erector spinae aponeurosis). This attaches firmly to the sacrum and iliac crest, providing a massive base of support for lifting.

2. The Lateral Leverage

Because the Iliocostalis is the most lateral column (farthest from the spine), it has the greatest leverage for Lateral Flexion (side bending). However, its oblique fiber orientation also makes it effective at controlling rotation eccentrically.

Physio Corner: Clinical Relevance

💪 Functional Fact: During a deadlift or squat, the Iliocostalis Lumborum works isometrically to keep the spine neutral. If you "round your back," you are putting these muscles in a stretched, weakened position, shifting the load to the passive ligaments and discs.

Palpation

Ask the patient to lie prone and lift their upper chest off the table. Palpate lateral to the lumbar spine (about 2-3 inches from the midline). The thick, rope-like muscle mass running vertically up from the iliac crest to the ribs is the Iliocostalis Lumborum.

⚠️ Clinical Pathology: Mechanical Low Back Pain
Strain of the Iliocostalis Lumborum is the most common cause of acute low back spasms ("throwing your back out"). This often happens during a combination of flexion and rotation (e.g., picking up a pencil while twisting).

Manual Muscle Testing (MMT)

We test the lumbar extensors as a group (Erector Spinae).

Testing Tip: Ensure the patient clears the umbilicus (belly button) from the table for a Grade 5. Stabilize the legs firmly!

Step-by-Step Procedure (Sorenson Test / Extensor Endurance)

Grade	Patient Action & Resistance
Grade 3, 4, 5 (Against Gravity)	Position: Prone (face down), hands behind head or at sides. Action: Patient extends the lumbar spine, lifting the chest and stomach off the table. Resistance: Applied to the upper thoracic spine, pushing down into flexion. Grade 5: Holds like a rock against strong pressure. Grade 3: Completes full range without resistance.
Grade 0, 1 (Palpation)	Action: Palpate lateral to the spine in the lumbar region. Cue: "Try to arch your back." Grade 1: Strong contraction felt in the paravertebral mass. Grade 0: No activity.

Frequently Asked Questions

Does it rotate the spine?

Yes, but primarily as a stabilizer. Unilaterally, it can assist in ipsilateral rotation, but its main unilateral job is lateral flexion. It mostly acts eccentrically to slow down rotation to the opposite side.

How is it different from Quadratus Lumborum?

Iliocostalis: Superficial, runs vertically, primary extensor.
Quadratus Lumborum (QL): Deep, runs obliquely/vertically between crest and 12th rib, primary hip hiker and lateral stabilizer.

Does it attach to the neck?

No. The Lumborum portion stops at the lower ribs. However, the muscle column continues upward as the Iliocostalis Thoracis and then the Iliocostalis Cervicis, forming a relay system up the back.

Test Your Knowledge: Iliocostalis Lumborum Quiz

1. The Iliocostalis Lumborum is part of which muscle group?

2. Where does the Iliocostalis Lumborum insert?

3. What is the primary bilateral action?

4. Which column of the Erector Spinae is the Iliocostalis?

5. Unilateral contraction causes:

6. The thick connective tissue covering the origin of this muscle is the:

7. Which nerve supplies the Iliocostalis?

8. "Throwing your back out" usually involves a strain during which combined motion?

9. Which muscle is Medial to the Iliocostalis?

10. True or False: Iliocostalis Lumborum originates from the Ribs.

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.
• Kendall, F. P. (2005). Muscles: Testing and Function, with Posture and Pain. 5th ed. Lippincott Williams & Wilkins.