Rectus abdominis

Musculus rectus abdominis

Also known as: abs, six-pack, linea alba, tendinous intersections

A long, strap-like vertical muscle on either side of the linea alba, enclosed in the rectus sheath. It flexes the trunk, tenses the abdominal wall and raises intra-abdominal pressure.

Overview

Rectus abdominis runs from the pubic crest and pubic symphysis to the xiphoid process and costal cartilages 5–7. It is wider and thinner above. It is crossed by usually three tendinous intersections, at the level of the xiphoid, the umbilicus and halfway between. These are fused to the anterior layer of the rectus sheath and produce the "six-pack" appearance in lean, muscular people. Each muscle is enclosed in the rectus sheath, formed by the aponeuroses of the three flat abdominal muscles. The two recti are separated in the midline by the linea alba, and their curved lateral borders form the linea semilunaris. The small triangular pyramidalis, present in ≈80% of people, lies in front of its lower end.

It is the main flexor of the trunk (sit-ups, curling forward, flexing the lumbar spine). It also compresses the abdomen, stabilises the pelvis during walking, and helps in forced expiration, coughing, defecation and labour. Segmental innervation by the thoraco-abdominal nerves (T7–T11) and the subcostal nerve (T12) enters from the lateral side, so a vertical incision along the lateral border denervates the muscle.

Location

Anterior abdominal wall, either side of the midline, from the pubis to the lower costal margin.

Parts & subdivisions

Tendinous intersections

Usually three transverse fibrous bands adherent to the anterior rectus sheath. They let the muscle act in segments and contain segmental vessels.

Linea alba

The midline fibrous raphe where the aponeuroses from both sides interlace, from the xiphoid to the symphysis. It is relatively avascular, which makes it a common surgical incision line.

Linea semilunaris

The curved lateral border of the rectus sheath, from the 9th costal cartilage to the pubic tubercle.

Rectus sheath

The aponeurotic compartment enclosing the muscle.

Anatomy

Origin

Pubic crest and pubic symphysis.

Insertion

Xiphoid process and the costal cartilages of ribs 5–7.

Action

Flexes the trunk; compresses the abdominal viscera; tilts the pelvis posteriorly; assists forced expiration.

Innervation

Thoraco-abdominal nerves (anterior rami of T7–T11) and the subcostal nerve (T12).

Nerve roots

T7–T12

Arterial supply

Superior epigastric artery (from the internal thoracic artery) and inferior epigastric artery (from the external iliac artery), which anastomose within the sheath; posterior intercostal branches laterally.

Relations

The epigastric vessels run on its posterior surface within the sheath. Below the arcuate line, only the transversalis fascia separates it from the peritoneum.

Clinical significance

Diastasis recti

Widening of the linea alba (>2 cm) between the recti, common after pregnancy and in newborns. A midline ridge appears on straining. It is not a true hernia because there is no fascial defect.

Rectus sheath haematoma

Rupture of the inferior epigastric artery, often in anticoagulated patients after coughing, produces a tender mass that does not cross the midline and stays palpable when the recti are tensed (Fothergill sign).

Inferior epigastric artery in surgery

It defines direct (medial) versus indirect (lateral) inguinal hernias. It can be injured by laparoscopic port placement. It is the pedicle of the DIEP and TRAM flaps used in breast reconstruction.