The transversus abdominis (also known as the transverse abdominal muscle) is the deepest of the flat muscles of the anterolateral abdominal wall, with fibers running transversely. Deep to this muscle lies a well-developed fascia, known as the transversalis fascia.

Origin: thoracolumbar fascia, anterior two-thirds of the iliac crest, inner surfaces of the lower six costal cartilages (cartilages of ribs 7 to 12), iliopectineal arch.

Insertion: aponeurosis of the internal abdominal oblique muscle ending in the linea alba; muscle fibers also insert into the pubic crest and pectineal line of the pubic bone.

Action: bilateral activation of this muscle participates in expiration and compresses abdominal contents, increasing the intra-abdominal pressure.

Innervation: anterior rami of the lower six thoracic spinal nerves (intercostal nerves T7 to T11, 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.

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Transversus abdominis muscle