Transversus abdominis

The body's natural corset

Abdomen · deepest flat muscle of the wall

The innermost of the three flat muscles of the abdominal wall, whose fibres run horizontally round the trunk like a corset from the back and the lower ribs to the midline in front. Its job is to compress the abdominal contents and to raise the pressure inside the abdomen, which helps to stabilise the spine and lower ribs. A plane between it and the internal oblique carries the nerves of the abdominal wall.

How to spot them

▸Deepest of the three flat abdominal muscles, with fibres running transversely, straight across the trunk
▸Arises from the lower six costal cartilages, the thoracolumbar fascia, the iliac crest and the lateral inguinal ligament
▸Its lower fibres join those of the internal oblique to form the conjoint tendon at the pubic bone
▸The plane between it and the internal oblique contains the nerves and vessels of the abdominal wall

What to know

The deepest of the three flat abdominal muscles, its fibres running horizontally round the body from the back to the front

Transversus abdominis — the body's natural corset.

Compresses the abdominal contents and raises the pressure inside the abdomen, as in forced coughing, vomiting or childbirth

Transversus abdominis — abdominal compression.

Its lower fibres fuse with those of the internal oblique to form a tendon attached to the pubic crest, reinforcing the back of the inguinal canal

Transversus abdominis — the conjoint tendon (falx inguinalis).

Local anaesthetic is injected into the plane between it and the internal oblique to numb the abdominal wall after surgery

Transversus abdominis — the TAP (transversus abdominis plane) block.

A tendon on its back attaches it to the lumbar vertebrae, forming a firm band round the loins

Transversus abdominis — its origin from the thoracolumbar fascia.

Physiotherapists teach patients to draw in the belly button to switch it on before lifting

Transversus abdominis — the abdominal drawing-in manoeuvre for core stability.

Key ideas

Its horizontal fibres pull like the strings of a corset, drawing the abdominal wall in and increasing intra-abdominal pressure without moving the spine much; that is why it is often called the core stabiliser.
It contracts a fraction of a second before the limb moves when a healthy person lifts an arm or leg; studies by Hodges and Richardson in the 1990s found this early timing delayed in people with low back pain, which prompted specific core-stability exercises.

Quick-fire facts

•Origin: the lateral third of the inguinal ligament, the anterior two-thirds of the iliac crest, the thoracolumbar fascia and the inner surfaces of the costal cartilages of ribs 7 to 12
•Insertion: the linea alba (through an aponeurosis), the xiphoid process and the pubic crest and pecten via the conjoint tendon
•Action: compresses the abdominal contents and increases intra-abdominal pressure; forced expiration
•Nerve supply: thoracoabdominal nerves (T7 to T11), the subcostal nerve (T12), and the iliohypogastric and ilioinguinal branches of L1
•Blood supply: lower posterior intercostal and subcostal arteries, superior and inferior epigastric, deep circumflex iliac and lumbar arteries
•The nerves and vessels of the wall run in the transversus abdominis plane, between it and the internal oblique
•The semilunar line, the lateral border of the rectus sheath, marks where its aponeurosis begins
•The conjoint tendon forms part of the posterior wall of the inguinal canal, strengthening the region of the superficial ring
•Above the arcuate line its aponeurosis passes behind the rectus abdominis; below that line it passes in front

Drill it — 6 cards

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Sources

TeachMeAnatomy — Transversus abdominis