The internal abdominal oblique muscle lies deep to the external oblique abdominal muscle on each side of the anterolateral abdominal wall. Its fibers run superomedially - perpendicular to the fibers of the external oblique, and its fibers are smaller and thinner in structure.

Origin: thoracolumbar fascia; anterior two-thirds of the iliac crest (between the origins of the external abdominal oblique and transversus abdominis muscles); iliopectineal arch, anterior superior iliac spine.

Insertion: inferior borders of the lower three or four ribs (usually, ribs 10 to 12); aponeurosis ending in linea alba; pubic crest and pectineal linea.

Action: bilateral activation of this muscle provides expiration and compresses abdominal contents, increasing the intra-abdominal pressure. Bilateral contractions may also flex the trunk. During unilateral activation, the muscle provides lateral flexion and rotation of the trunk to the same side (ipsilateral).

Innervation: ventral rami of the lower six thoracic spinal nerves (namely, the ventral rami of the intercostal nerves (T7 - T11) and the subcostal nerve (T12)), and the first lumbar spinal nerve (L1).

Blood supply: lower posterior intercostal arteries, subcostal artery, lumbar arteries, superior and inferior epigastric arteries, superficial and deep circumflex iliac arteries.