The internal oblique lies deep to the external oblique with fibres running at 90 degrees to it (superomedially — 'fanning' pattern). With the external oblique it forms the oblique muscle pair that produces rotation. The internal oblique's conjoint tendon with the transversus abdominis reinforces the posterior inguinal canal wall medial to the deep ring in direct inguinal hernia repair.
| Origin | Thoracolumbar fascia, anterior two-thirds of the iliac crest, and lateral inguinal ligament |
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| Insertion | Lower three costal cartilages, xiphoid, linea alba (via aponeurosis), and conjoint tendon to pubic crest |
| Nerve Supply | Intercostal nerves T8-T12, iliohypogastric and ilioinguinal nerves (L1) |
| Blood Supply | Lower intercostal and lumbar arteries |
| Actions | Ipsilateral rotation (works with the opposite external oblique for rotation); Ipsilateral lateral flexion; Bilateral contraction: trunk flexion, abdominal compression |
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Internal oblique strain from explosive trunk rotation produces a deep lateral abdominal pain. In inguinal hernia anatomy, the internal oblique and transversus abdominis muscles form the muscular arch that ideally sweeps over the inguinal canal to reinforce its posterior wall — the basis of the Shouldice repair technique.
Deep to the external oblique and not independently palpable — assessed via ultrasound cross-section of the lateral abdominal wall.
Tear of the internal oblique from the lower costal cartilage in cricket and baseball fast bowlers producing acute lateral thoracic pain — the most common injury in fast bowlers.
