Origin

• Anterior iliac crest
• Thoracolumbar fascia
• Lateral two-thirds of the inguinal ligament
• Iliopsoas fascia

Insertion

The internal abdominal oblique muscle is a broad thin muscular sheet found on the lateral side of the abdomen. As its name suggests, the direction of its fibers are obliquely oriented, perpendicular to the external abdominal oblique muscle fibers.

The three distinct layers of the lateral abdominal wall, going from superficial to deep, are: the external abdominal oblique muscle, internal abdominal oblique muscle and TVA.

This internal abdominal oblique muscle has multiple sites of origin. According to their origin, the muscle fibers can be divided into the anterior, lateral and posterior fibers:

• Anterior fibers
  Rises from the lateral two-thirds of the superior surface of the inguinal ligament, forming a common attachment with the iliac fascia.

• Lateral fibers
  Originate from the anterior two-thirds of iliac crest and extend into an aponeurosis that contributes to the formation of the rectus sheath and inserts at the linea alba.

• Posterior fibers
  Originate from the posterior end of the iliac crest and the thoracolumbar fascia. The fibers then ascend superiorly and laterally and insert into the lower three or four ribs and their cartilages, merging with the internal intercostals.

Along with the other muscles of the abdominal wall, the tonic contraction of the internal abdominal oblique muscle plays an important role in protecting and supporting movements of the trunk, maintaining normal abdominal tension, and increasing intra-abdominal pressure.

It performs two main functions. Firstly, it acts as an antagonist to the diaphragm, because, as an accessory muscle of respiration, it contributes to the reduction of the volume of the chest cavity during exhalation.

Secondly, its contraction causes ipsilateral rotation and side-flexion.

In rotational trunk movements, this internal oblique abdominal muscle contracts on the opposite side of the external oblique abdominal muscle. For example, the right internal oblique and the left external oblique muscles contract as the torso flexes and rotates to bring the left shoulder towards the right hip.

Bilateral Contraction

• Trunk Flexion
  Assists in bending the torso forward.
• Compression of Abdominal Viscera
  Supports internal organs and maintains intra-abdominal pressure.
• Expiration Assistance
  Acts as an antagonist to the diaphragm, reducing chest cavity volume during exhalation.

Unilateral Contraction

• Ipsilateral Trunk Lateral Flexion
  Bends the torso sideways toward the contracting side.
• Ipsilateral Trunk Rotation
  Rotates the trunk toward the same side, working with the opposite external oblique for smooth rotational movement.

Core Stabilization and Postural Support

• Works with the transverse abdominis to stabilize the spine and pelvis.
• Prevents excessive spinal movement, reducing lower back strain.

Intra-Abdominal Pressure Regulation

• Plays a key role in compressing the abdominal contents, aiding in forced expiration (deep exhalation), coughing and sneezing, micturition (urination), defecation, vomiting, and the final stages of childbirth.

• Pelvic and Lower Abdominal Support
  
  • Works in coordination with the pelvic floor muscles and transverse abdominis to stabilize the pelvis.
  • Helps maintain pelvic alignment, reducing strain during walking, running, and lifting.