The internal oblique has fibres running roughly perpendicular to the external oblique, with most fibres directed upward and medially. Unlike the external oblique which produces contralateral trunk rotation, the internal oblique produces ipsilateral rotation, the right internal oblique rotates the trunk to the right. Together with the contralateral external oblique, each internal oblique participates in the rotational coupling that is essential for throwing, striking, and gait.
| Origin | Thoracolumbar fascia, anterior two-thirds of the iliac crest, and lateral two-thirds of the inguinal ligament |
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| Insertion | Lower three or four costal cartilages, linea alba via the aponeurosis, pubic crest, and pectineal line |
| Nerve Supply | Lower five intercostal nerves (T8 through T12); Subcostal (T12); Iliohypogastric and ilioinguinal nerves (L1) |
| Blood Supply | Lower posterior intercostal arteries; Superior and inferior epigastric arteries |
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Paired with the contralateral external oblique, the internal oblique contributes to the functional diagonal slings (anterior oblique sling) that transfer forces between the upper and lower extremities during functional movement patterns.
Internal oblique strains are common in throwing athletes and golfers from the powerful rotational loading during the follow-through phase. The internal oblique and its aponeurosis form the roof of the inguinal canal, and its lower fibres fuse with the transversus abdominis to form the conjoint tendon (falx inguinalis) that inserts on the pubic crest and is repaired in inguinal herniorrhaphy.
The internal oblique is palpable in the lateral abdominal wall between the iliac crest and the rib cage, deep to the external oblique. Its fibres run perpendicular to the external oblique and become firm during resisted ipsilateral trunk rotation.
Muscle tear from explosive trunk rotation in throwing athletes producing lateral abdominal pain that worsens with ipsilateral rotation and is distinguished from external oblique strain by the direction of rotation that provokes it.
