Internal Oblique: Origin, Insertion, Action & "Same Side" Rotation

The Internal Oblique is the intermediate layer of the lateral abdominal muscles, lying deep to the External Oblique and superficial to the Transversus Abdominis. While the External Oblique rotates the trunk to the opposite side, the Internal Oblique is the primary rotator to the same side.

[Image of Internal Oblique muscle anatomy]

Quick Anatomy Snapshot

Layer	Intermediate (Middle Layer of Abdominal Wall).
Origin (Proximal)	Thoracolumbar Fascia, Anterior 2/3 of Iliac Crest, and Lateral 2/3 of Inguinal Ligament.
Insertion (Distal)	Inferior borders of Ribs 10-12, Linea Alba (via aponeurosis), and Pecten Pubis (via Conjoint Tendon).
Nerve Supply	Thoracoabdominal Nerves (T7-T11), Subcostal (T12), and Iliohypogastric/Ilioinguinal (L1).
Primary Actions	Bilateral: Flexion of the trunk; Compression of abdominal viscera (forced expiration). Unilateral: Ipsilateral Rotation (turns trunk to same side) and Lateral Flexion.

Deep Dive: The "Back Pocket" Muscle

The fiber direction is the key to distinguishing the Internal from the External Oblique.

1. "Hands in Back Pockets"

The fibers of the Internal Oblique run Superomedially (Up and In). This opposes the "Hands in Front Pockets" direction of the External Oblique. Imagine reaching across your chest to put your hand in the opposite back pocket—that aligns with the Internal Oblique fibers.

2. The Conjoint Tendon

The lowest fibers of the Internal Oblique arch over the spermatic cord (or round ligament) and fuse with the tendon of the Transversus Abdominis to form the Conjoint Tendon (Falx Inguinalis). This structure inserts onto the pubic crest and is a critical reinforcement against direct inguinal hernias.

Physio Corner: Clinical Relevance

💪 Functional Fact: Trunk rotation is a team effort. To rotate to the Left, you use your Left Internal Oblique (Ipsilateral) and your Right External Oblique (Contralateral). They form a diagonal muscular sling across the torso.

Palpation

Palpation is difficult because it is covered by the External Oblique. However, you can feel it tensing deep to the External Oblique just medial to the ASIS (Anterior Superior Iliac Spine) when the patient rotates their trunk toward the side being palpated.

⚠️ Clinical Pathology: Cremasteric Reflex
The Cremaster Muscle, which elevates the testes, is actually a derivation of the Internal Oblique muscle fibers. It loops down through the inguinal canal. Stroking the inner thigh triggers this reflex (L1-L2), causing the cremaster (and indirectly the Internal Oblique) to contract.

Manual Muscle Testing (MMT)

We test trunk rotation to assess the obliques.

Testing Tip: To isolate the Right Internal Oblique, ask the patient to bring their Right shoulder toward their Right hip (side crunch) or rotate their Left shoulder toward their Right hip.

Step-by-Step Procedure (Trunk Rotation)

Grade	Patient Action
Grade 5 (Normal)	Position: Supine, hands behind head. Action: Patient flexes and rotates trunk to the RIGHT (bringing Left elbow to Right knee). Muscles Tested: Right Internal Oblique & Left External Oblique. Result: Inferior angle of scapula clears the table.
Grade 4 (Good)	Position: Supine, arms crossed over chest. Action: Same rotation. Result: Scapula clears the table.
Grade 3 (Fair)	Position: Supine, arms extended forward. Action: Same rotation. Result: Scapula clears the table.

Frequently Asked Questions

Does it help with posture?

Yes. Bilateral contraction helps flatten the lumbar curve (posterior pelvic tilt) and compresses the abdominal contents to create intra-abdominal pressure, which stabilizes the spine.

What lies deep to the Internal Oblique?

The Transversus Abdominis muscle lies immediately deep to it. The neurovascular plane (thoracoabdominal nerves) runs between the Internal Oblique and Transversus Abdominis.

Is it involved in sports hernias?

Yes. Tears in the Conjoint Tendon (formed by Internal Oblique and Transversus Abdominis) are a common cause of "Sports Hernia" or Athletic Pubalgia, leading to chronic groin pain.

Test Your Knowledge: Internal Oblique Quiz

1. What is the fiber direction of the Internal Oblique?

2. Unilateral contraction of the Right Internal Oblique causes rotation to:

3. The Conjoint Tendon is formed by the Internal Oblique and which other muscle?

4. Which muscle is derived from the Internal Oblique fibers?

5. The nerves supplying the Internal Oblique run between which two layers?

6. Where does the Internal Oblique originate?

7. Which nerve contributes to the supply of the lower fibers of Internal Oblique?

8. The Internal Oblique inserts into which ribs?

9. Which muscle works diagonally with the Right Internal Oblique for rotation?

10. True or False: The Internal Oblique helps in forced expiration.

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.