Transversus Abdominis

The transversus abdominis is the deepest of the three flat muscles of the anterior and lateral abdominal wall. It lies beneath the internal oblique and plays a central role in abdominal compression and core stabilization. Unlike the other abdominal muscles, its fibers run horizontally, providing a girdle-like effect that stabilizes the trunk and supports the spine. It is particularly important in functional movements, breathing mechanics, and maintaining intra-abdominal pressure. Though not directly involved in trunk rotation or flexion, its role in dynamic core stability makes it essential in both anatomical and clinical contexts.

Structure

The transversus abdominis is a broad, thin muscle with fibers oriented transversely. It forms the innermost muscular layer of the lateral abdominal wall and transitions medially into a broad aponeurosis that helps form the rectus sheath.

Origin

• Internal surfaces of costal cartilages of ribs 7–12
• Thoracolumbar fascia
• Iliac crest (anterior two-thirds of inner lip)
• Lateral third of the inguinal ligament

Insertion

• Linea alba via aponeurosis
• Pubic crest
• Pecten pubis via the conjoint tendon

Fiber Direction

The muscle fibers run transversely (horizontally) across the abdominal wall. This is unique compared to the inferomedial fibers of the external oblique and the superomedial fibers of the internal oblique.

Layering Relationships

Layer	Position Relative to Transversus Abdominis
Skin, Camper's fascia, Scarpa's fascia	Superficial
External oblique	Superficial
Internal oblique	Superficial (immediate)
Transversalis fascia	Deep
Parietal peritoneum	Deep to fascia

The neurovascular plane of the abdominal wall lies between the internal oblique and transversus abdominis, where intercostal nerves and vessels travel.

Function

The transversus abdominis is not directly involved in visible trunk movement like bending or rotation. Instead, its primary functions include:

• Core stabilization: Activates before limb movement to stabilize the lumbar spine and pelvis.
• Compression of abdominal contents: Increases intra-abdominal pressure, assisting in forced expiration, defecation, urination, and childbirth.
• Trunk rigidity: Acts like a corset, drawing in the abdominal wall.
• Postural support: Works with multifidus and pelvic floor muscles for upright posture and spinal alignment.

Contribution to Rectus Sheath

The aponeurosis of the transversus abdominis contributes to the rectus sheath but its location changes based on the level:

• Above the arcuate line: The aponeurosis passes posterior to the rectus abdominis, forming the posterior layer of the sheath.
• Below the arcuate line: It joins the aponeuroses of the external and internal obliques anterior to the rectus abdominis. No posterior sheath exists at this level.

This anatomical configuration is important during surgical incisions and in understanding hernia formations.

Conjoint Tendon

At its inferior-medial end, fibers of the transversus abdominis and internal oblique merge to form the conjoint tendon (falx inguinalis), which inserts into the pubic crest and pecten pubis.

The conjoint tendon supports the medial posterior wall of the inguinal canal and is a key structure in resisting direct inguinal herniation.

Relationship to the Inguinal Canal

• The transversus abdominis arches above the inguinal canal but does not contribute muscle fibers to the canal itself.
• Its aponeurotic fibers, however, contribute to the conjoint tendon which reinforces the posterior wall medially.

Nerve Supply

The transversus abdominis is innervated segmentally by:

• Lower six thoracoabdominal nerves (T7–T11)
• Subcostal nerve (T12)
• Iliohypogastric and ilioinguinal nerves (L1)

These nerves travel within the plane between the transversus abdominis and the internal oblique, and they supply both motor and sensory innervation.

Blood Supply

Arterial supply is drawn from several sources:

• Posterior intercostal arteries (7–11)
• Subcostal artery
• Superior and inferior epigastric arteries
• Deep circumflex iliac artery

Venous drainage mirrors the arterial distribution and drains into thoracic, lumbar, and iliac veins.

Embryological Origin

The transversus abdominis, like other abdominal muscles, is derived from myotomes of the paraxial mesoderm. During development, it migrates to the lateral body wall and takes its innervation from the segmental spinal nerves that accompany it.

Clinical Significance

• Core training and rehabilitation: Transversus abdominis activation is emphasized in physiotherapy and athletic training for low back pain and pelvic stabilization.
• Surgical relevance: During laparoscopic or open abdominal procedures, care must be taken not to damage neurovascular structures that lie just deep to the muscle.
• Spinal support: A weak or dysfunctional transversus abdominis is associated with reduced spinal stability and increased risk of injury.
• Hernia prevention: The conjoint tendon, formed partly by this muscle, is a key structure in reinforcing the inguinal region against herniation.

Unlike more superficial muscles, injury to the transversus abdominis is less common in isolation but may occur in conjunction with internal oblique or fascia damage during high-impact trauma or abdominal surgeries.