Iliocostalis Thoracis: Origin, Insertion, Action & Posture

The Iliocostalis Thoracis is the intermediate segment of the Iliocostalis muscle column. It acts as a "relay station" in the back, connecting the lower ribs to the upper ribs. It is a key muscle for maintaining upright thoracic posture and preventing the "slouched" kyphotic position.

[Image of Iliocostalis Thoracis anatomy]

Quick Anatomy Snapshot

Group	Erector Spinae (Lateral Column).
Origin (Proximal)	Superior borders of the angles of the Lower 6 Ribs (Ribs 7-12), medial to the insertion of Iliocostalis Lumborum.
Insertion (Distal)	Superior borders of the angles of the Upper 6 Ribs (Ribs 1-6) and the Transverse Process of C7.
Nerve Supply	Lateral branches of the Posterior Rami of thoracic spinal nerves.
Primary Actions	Bilateral: Extension of the thoracic spine. Unilateral: Lateral flexion (side bending) of the thorax. Respiration: Assists in forced expiration by depressing ribs (controversial/minor role).

Deep Dive: The "Rib-to-Rib" Muscle

While the Iliocostalis Lumborum connects the hip to the ribs, and the Cervicis connects ribs to the neck, the Thoracis segment connects ribs to ribs.

1. The Relay System

The Erector Spinae functions like a relay race. The Iliocostalis Lumborum travels up and attaches to the lower ribs. The Iliocostalis Thoracis originates from those same lower ribs (medial to the Lumborum attachment) and travels higher to the upper ribs. This overlapping arrangement allows for a seamless transfer of force from the pelvis to the neck.

2. Angle to Angle

The specific attachment point is the Angle of the Rib (the sharpest curve of the rib bone posteriorly). This lateral position gives the muscle excellent leverage for Lateral Flexion and rotational control.

Physio Corner: Clinical Relevance

💪 Functional Fact: This muscle is the enemy of the "Desk Slouch." When you hunch over a computer, the Iliocostalis Thoracis becomes overstretched and weak (locked long). Strengthening it is essential for correcting excessive Thoracic Kyphosis (hunchback).

Palpation

Locate the spinous processes of the mid-back. Move laterally about 3-4 fingers width, past the Longissimus muscle. Palpate over the rib angles (under the scapula/rhomboids). Ask the patient to lift their chest off the table. The thin vertical bands tensing over the ribs are the Iliocostalis Thoracis.

⚠️ Clinical Pathology: Rib Dysfunction
Because it attaches directly to the ribs, hypertonicity (tightness) in the Iliocostalis Thoracis can restrict rib movement. This may mimic "stuck ribs" or costovertebral joint dysfunction, causing sharp pain upon deep inhalation or twisting.

Manual Muscle Testing (MMT)

Testing the thoracic extensors.

Testing Tip: The patient must lift the upper trunk (chest) off the table. To isolate the thoracic region from the lumbar, stabilize the lower back/pelvis firmly.

Step-by-Step Procedure (Oxford Scale)

Grade	Patient Action & Resistance
Grade 3, 4, 5 (Against Gravity)	Position: Prone (face down), hands behind head (harder) or at sides (easier). Action: Patient extends the thoracic spine, lifting the sternum off the table. Resistance: Applied to the upper thoracic spine (between scapulae), pushing down. Grade 3: Clears sternum (xiphoid process) from table. Grade 4/5: Holds against moderate/strong resistance.
Grade 0, 1 (Palpation)	Action: Palpate lateral to the Longissimus, over the posterior ribs. Cue: "Try to lift your chest." Grade 1: Contraction felt. Grade 0: No activity.

Frequently Asked Questions

Is it deep to the Rhomboids?

Yes. In the upper back, the Iliocostalis Thoracis lies deep to the Rhomboids and the Trapezius. You have to palpate "through" these superficial muscles to feel the erector spinae.

Does it help with breathing?

It helps stabilize the ribs against the pull of the diaphragm. In forced expiration, it can help depress the ribs, but its primary role is postural extension.

Does it attach to the vertebrae?

Mostly no. Its primary attachments are rib-to-rib. However, its most superior fibers often attach to the Transverse Process of C7, bridging the transition to the neck.

Test Your Knowledge: Iliocostalis Thoracis Quiz

1. The Iliocostalis Thoracis originates from:

2. Where does it insert?

3. What is the primary action of the Iliocostalis Thoracis?

4. The Iliocostalis is which column of the Erector Spinae?

5. Which muscle lies Medial to the Iliocostalis Thoracis?

6. Unilateral contraction results in:

7. This muscle is typically weak/long in which posture?

8. Which nerve supplies the Iliocostalis Thoracis?

9. The Iliocostalis Thoracis is deep to which muscles?

10. True or False: This muscle connects ribs to 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.