The external oblique is the largest and most superficial of the flat abdominal muscles, with fibres running downward and medially in the same direction as the fingers when placed in a trouser pocket. Its contralateral trunk rotation action means the right external oblique rotates the trunk to the left, working in synergy with the left internal oblique as an oblique force couple. The inguinal ligament is the rolled-under free lower border of its aponeurosis, making the external oblique the muscle whose aponeurosis forms the inguinal canal.
| Origin | External surfaces of ribs 5 through 12 |
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| Insertion | Linea alba via the aponeurosis, pubic crest, pubic tubercle, anterior half of the iliac crest, and the inguinal ligament (lower free border of the aponeurosis) |
| Nerve Supply | Lower six intercostal nerves (T7 through T12); Subcostal nerve (T12) |
| Blood Supply | Lower posterior intercostal arteries; Subcostal artery |
| Actions | Contralateral rotation of the trunk; Ipsilateral lateral flexion of the trunk; Compression of the abdominal contents; Forced expiration; Flexion of the trunk |
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The external oblique is a powerful trunk rotator and the primary muscle of the anterior oblique sling that connects the contralateral shoulder to the hip girdle, transmitting force across the midline during throwing and striking movements.
External oblique strains from explosive trunk rotation are common in throwing athletes, golfers, and tennis players. The inguinal ligament, formed by the external oblique aponeurosis, is the superior boundary of the femoral triangle. The superficial inguinal ring, an opening in the external oblique aponeurosis, is the exit of the inguinal canal and the site where indirect inguinal hernias emerge.
The external oblique is palpable in the anterolateral abdominal wall with fibres running downward and medially, becoming clearly firm during resisted contralateral trunk rotation and during a sit-up with trunk rotation.
Muscle tear from explosive contralateral trunk rotation in throwing sports producing anterolateral abdominal pain that worsens with contralateral rotation and is reproduced by resisted trunk rotation against the affected side.
Protrusion through the superficial inguinal ring or directly through the internal oblique and transversus abdominis (direct), producing an inguinal bulge that worsens with Valsalva and requires surgical repair when symptomatic.
