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

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

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

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