Its fibers run perpendicular to the external oblique muscle, beginning in the thoracolumbar fascia of the lower back, the anterior 2/3 of the iliac crest (upper part of hip bone) and the lateral half of the inguinal ligament.

Firstly as an accessory muscle of respiration, it acts as an antagonist (opponent) to the diaphragm, helping to reduce the volume of the chest cavity during exhalation.

When the diaphragm contracts, it pulls the lower wall of the chest cavity down, increasing the volume of the lungs which then fill with air.

Conversely, when the internal obliques contract they compress the organs of the abdomen, pushing them up into the diaphragm which intrudes back into the chest cavity reducing the volume of the air-filled lungs, producing an exhalation.

It acts with the external oblique muscle of the opposite side to achieve this torsional movement of the trunk.