External Oblique: Origin, Insertion, Action & The "Pocket" Muscle

The External Oblique is the largest and most superficial of the three flat lateral abdominal muscles. It wraps around the torso like a corset. Its fibers run inferomedially (down and in), mimicking the direction of putting your hands into your front pockets.

[Image of External Oblique muscle anatomy]

Quick Anatomy Snapshot

Origin (Proximal)	External surfaces of Ribs 5-12 (interdigitating with Serratus Anterior and Latissimus Dorsi).
Insertion (Distal)	Linea Alba, Pubic Tubercle, and the Anterior half of the Iliac Crest.
Nerve Supply	Thoracoabdominal Nerves (T7-T11) and Subcostal Nerve (T12).
Blood Supply	Lower posterior intercostal arteries, Subcostal artery, Deep circumflex iliac artery.
Primary Actions	Bilateral: Flexion of the trunk; Compression of abdominal viscera (forced expiration). Unilateral: Contralateral Rotation (turns trunk to opposite side) and Ipsilateral Lateral Flexion (side bending).

Deep Dive: The "Pocket" Muscle

The External Oblique is defined by the direction of its fibers and its connective tissue attachments.

1. "Hands in Pockets"

The fibers run Down and In. This is the easiest way to distinguish it from the Internal Oblique (which runs Up and In) and Transversus Abdominis (which runs horizontally). If you place your hands in your pockets, your fingers align with the External Oblique fibers.

2. The Inguinal Ligament

The inferior border of the External Oblique aponeurosis thickens and folds back on itself between the ASIS (Anterior Superior Iliac Spine) and the Pubic Tubercle. This thickened fold creates the Inguinal Ligament, a crucial landmark in groin anatomy and hernia repair.

Physio Corner: Clinical Relevance

💪 Functional Fact: The External Oblique works in a "diagonal chain" with the Internal Oblique of the opposite side. For example, when you rotate your trunk to the Right, you are using your Left External Oblique and your Right Internal Oblique.

Palpation

Have the patient lie supine with knees bent. Ask them to lift their head and shoulder off the table and twist to one side. Palpate the lateral abdominal wall on the side opposite the twist. The superficial sheet tensing over the ribs is the External Oblique.

⚠️ Clinical Pathology: Sports Hernia (Athletic Pubalgia)
While not a true "hernia" (hole), this is often a strain or tear of the soft tissue in the groin. It frequently involves a disruption of the External Oblique aponeurosis near the inguinal ring, often caused by repetitive twisting and kicking (soccer, hockey).

Manual Muscle Testing (MMT)

Abdominal testing differs from limb testing; we use trunk movement against gravity to determine the grade.

Testing Tip: Look for the umbilicus to deviate. If the muscles are imbalanced, the belly button will be pulled toward the stronger quadrant (Beevor's Sign).

Step-by-Step Procedure (Trunk Rotation)

Grade	Patient Action
Grade 5 (Normal)	Position: Supine, hands behind head (fingers interlocked). Action: Patient flexes and rotates trunk, lifting the scapula off the table (bringing right elbow toward left knee). Result: Clears the inferior angle of the scapula from the table.
Grade 4 (Good)	Position: Supine, arms crossed over chest. Action: Patient flexes and rotates trunk. Result: Clears the inferior angle of the scapula.
Grade 3 (Fair)	Position: Supine, arms extended forward (reaching towards knees). Action: Patient flexes and rotates trunk. Result: Scapula clears the table.

Frequently Asked Questions

What is the "Adonis Belt"?

The "Adonis Belt" or "Apollo's Belt" refers to the visible grooves formed by the Inguinal Ligament (the thickened lower edge of the External Oblique aponeurosis) in people with low body fat.

Does it help with breathing?

Yes, but only during forced expiration (like coughing, blowing up a balloon, or shouting). It contracts to compress the abdomen, pushing the diaphragm up to expel air. It is relaxed during quiet breathing.

Does the External Oblique rotate to the same side?

No. It performs Contralateral Rotation. The Right External Oblique turns the torso to the Left. The Internal Oblique performs Ipsilateral (same side) rotation.

Test Your Knowledge: External Oblique Quiz

1. What is the direction of the External Oblique fibers?

2. The Inguinal Ligament is formed by the aponeurosis of which muscle?

3. Unilateral contraction of the Right External Oblique produces:

4. What nerve supplies the External Oblique?

5. Where does the External Oblique originate?

6. Which muscle lies deep to the External Oblique?

7. What is the primary role of the External Oblique in respiration?

8. The External Oblique inserts into which structure at the midline?

9. To test Grade 5 strength for rotation, where are the hands placed?

10. True or False: The External Oblique aids in defecation and childbirth.

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.