Internal oblique
Obliquus internus abdominis
Pronunciation: in-TER-nal oh-BLEEK
For beginners
Hidden beneath the external oblique, this muscle's fibers run the other diagonal way. It teams up with the external oblique on the OTHER side to twist your trunk.
Show the advanced explanation
Because its fibers run perpendicular to the external oblique's, the internal oblique also contributes strongly to anti-rotation bracing, not just active rotation.
Structure & function
- Location
- A deeper ab muscle beneath the external oblique, with fibers running diagonally the opposite direction (up and inward).
- Origin
- Iliac crest and inguinal ligament.
- Insertion
- Cartilage of the lower ribs and the linea alba.
- Main action(s)
- Flexes and laterally flexes the trunk; rotates the trunk to the SAME side when acting alone.
- Joint(s) crossed
- Intervertebral joints of the lumbar and lower thoracic spine
- Movement plane
- transverse (rotation); frontal (lateral flexion)
- Innervation
- Intercostal nerves (T7–T12) and the iliohypogastric/ilioinguinal nerves
- In daily movement
- Twisting and bracing, working as a same-side rotation partner to the opposite external oblique.
- In exercise
- Co-contracts with the transversus abdominis during bracing; drives rotation together with the opposite-side external oblique.
- Commonly confused with
- Its same-side rotation action is often mixed up with the external oblique's opposite-side action — they are mirror-image partners across the body.
- Common misconceptions
- It is not simply 'a smaller external oblique' — its fiber direction gives it a distinct, complementary rotation role.
Exercises that use this muscle
As a primary mover
⚠ Safety note
Same general precautions as other abdominal muscles: build twisting load gradually.
Sources
“The internal oblique flexes and rotates the vertebral column, rotating the trunk to the same side.”
Anatomy and Physiology 2e — OpenStax, Rice University · Ch. 11.3 Axial Muscles of the Abdominal Wall, and Thorax
Peer-reviewed textbook · License: CC BY-NC-SA 4.0 · Last checked 2026-07-19
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