Muscles of the Abdominal Wall

The muscles of the abdominal wall are a group of core muscles that surround and protect the abdominal cavity. These muscles contribute to various essential functions including trunk movement, posture maintenance, breathing, and generation of intra-abdominal pressure. They are arranged in distinct layers and include a combination of broad, flat muscles and vertically oriented strap-like muscles. Together, they form a dynamic and supportive sheath that both encases the abdominal organs and assists with functional tasks such as coughing, defecation, and childbirth.

Overview and Grouping

There are five principal muscles that make up the abdominal wall. They can be categorized based on their orientation and position:

Muscle	Type	Location
External Oblique	Flat	Superficial lateral
Internal Oblique	Flat	Intermediate lateral
Transversus Abdominis	Flat	Deepest lateral
Rectus Abdominis	Vertical (strap-like)	Anterior midline
Pyramidalis	Small, triangular	Anterior midline (inferior)

The three flat muscles are layered laterally and their aponeuroses contribute to the rectus sheath enclosing the vertically oriented rectus abdominis.

Individual Muscles

External Oblique

The external oblique is the most superficial of the lateral abdominal muscles. It has broad, flat fibers that run inferomedially, resembling the orientation of fingers in a "hands-in-pockets" position.

• Origin: Outer surfaces of ribs 5–12
• Insertion: Linea alba, iliac crest, and pubic tubercle via aponeurosis
• Function: Trunk rotation, lateral flexion, abdominal compression

Internal Oblique

Located beneath the external oblique, the internal oblique fibers run superomedially, nearly perpendicular to the external oblique.

• Origin: Thoracolumbar fascia, iliac crest, lateral half of the inguinal ligament
• Insertion: Lower ribs, linea alba, pubic crest, and pecten pubis
• Function: Same as external oblique; works in opposition for trunk rotation

Transversus Abdominis

This is the innermost flat muscle with fibers running horizontally. It acts primarily as a compressor of abdominal contents.

• Origin: Costal cartilages 7–12, thoracolumbar fascia, iliac crest, inguinal ligament
• Insertion: Linea alba, pubic crest, pecten pubis
• Function: Compresses abdominal contents; provides core stability

Rectus Abdominis

This is a long, strap-like vertical muscle that runs along the anterior abdominal wall. It is segmented by tendinous intersections and enclosed in the rectus sheath.

• Origin: Pubic symphysis and pubic crest
• Insertion: Xiphoid process and costal cartilages of ribs 5–7
• Function: Flexes vertebral column, compresses abdomen

Pyramidalis

A small triangular muscle found anterior to the lower part of the rectus abdominis in some individuals.

• Origin: Pubic crest
• Insertion: Linea alba
• Function: Tenses the linea alba (minor contribution)

Rectus Sheath and Linea Alba

The flat muscles of the abdominal wall contribute aponeuroses that surround the rectus abdominis and form the rectus sheath. This sheath varies in composition above and below the arcuate line.

• Above arcuate line: Internal oblique splits around rectus abdominis; transversus goes posterior
• Below arcuate line: All aponeuroses pass anterior to rectus; only transversalis fascia lies posteriorly

The linea alba is a fibrous median line extending from the xiphoid process to the pubic symphysis. It forms the midline raphe where the left and right aponeuroses meet.

Function

The muscles of the abdominal wall work in coordination to perform several vital functions:

• Support: Provide anterior and lateral support to abdominal contents
• Movement: Flexion, lateral bending, and rotation of the trunk
• Intra-abdominal pressure: Aid in urination, defecation, parturition, and lifting
• Respiration: Assist in forced expiration (e.g., coughing, sneezing)
• Posture: Stabilize the pelvis and lumbar spine

Nerve Supply

All the muscles of the abdominal wall are supplied by the following segmental nerves:

• Thoracoabdominal nerves (T7–T11)
• Subcostal nerve (T12)
• Iliohypogastric and ilioinguinal nerves (L1)

Blood Supply

The arterial supply is derived from:

• Superior and inferior epigastric arteries
• Lower intercostal arteries
• Deep circumflex iliac artery

Venous drainage follows the arterial supply, with connections to the portal and systemic systems via periumbilical veins.

Clinical Significance

• Hernias: Weaknesses or defects in the muscular wall can lead to inguinal, umbilical, or incisional hernias.
• Muscle tears: Common in athletes during sudden twisting or lifting motions.
• Diastasis recti: A condition where the rectus abdominis muscles separate, often seen in postpartum women.
• Surgical importance: The muscular planes are used for access in procedures like laparotomies or C-sections.