External abdominal oblique muscle
The external abdominal oblique muscle is the largest and most superficial flat muscle in the abdominal wall. Its fibers run in an inferomedial direction.
Origin: muscular slips arising from the outer surfaces of ribs 5 to 12.
Action: during bilateral contractions, the muscles compress abdominal contents, increase the intra-abdominal pressure and participate in the expiration; bilateral contractions also provide flexion of the trunk. Unilateral activation of this muscle provides lateral flexion of the trunk to the same side (ipsilateral). Unilateral contractions may also provide contralateral trunk rotation (to the opposite side).
Innervation: ventral rami of the lower six thoracic spinal nerves, which include the ventral rami of the intercostal nerves (T7 - T11) and the subcostal nerve (T12).