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Internal Oblique Muscle Anatomy

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Origin
  • Thoracolumbar fascia
  • Iliac crest
  • Inguinal ligament
Insertion
  • Inferior margins of tenth to twelfth ribs and adjacent costal cartilages
  • Linea alba
  • Pectineal line of the pubis (pecten pubis)
Action
  • Lateral flexion of the torso (unilateral contraction)
  • Rotation of the torso (unilateral contraction)
  • Flexion of the torso (bilateral contraction)
  • Can assist in increasing intra-abdominal pressure
Innervation
  • Anterior rami of intercostal nerves (T7-T11)
  • Subcostal nerve (T12)
  • Iliohypogastric nerve (L1)
  • Ilioinguinal nerve (L1)
Blood Supply
  • Lower posterior intercostal arteries and subcostal arteries
  • Superior and inferior epigastric arteries
  • Superficial and deep circumflex iliac arteries
  • Posterior lumbar arteries
Last updated: Estimated Reading Time: 7 min

Location & Overview

The internal oblique muscle is located deep to the external oblique muscle and superficial to the transversus abdominis muscle (i.e., between these two muscles). Its fibers are obliquely oriented, hence its name. The internal oblique is much thinner and smaller than the external oblique. Like the external oblique, it is a flat, sheet-like muscle that wraps around the abdomen. The internal oblique is one of the five abdominal muscles. The other four are the external oblique, rectus abdominis, transversus abdominis, and pyramidalis.[1,2,3]

internal oblique anterior view
Here, we can see the internal oblique from an anterior view.
internal oblique anterolateral view
Here, we can see the internal oblique from an anterolateral view.
internal oblique lateral view
Here, we can see the internal oblique from a lateral view.
internal oblique posterior view
Here, we can see the internal oblique from a posterior view.
internal oblique highlighted in red
Here, we can see the internal oblique highlighted in red among the other muscles of the body. Because the internal oblique is a relatively deep muscle, more superficial muscles such as the rectus abdominis and the external oblique have been removed to fully reveal the internal oblique. The muscle underlying the internal oblique here is the transversus abdominis.
internal oblique muscle with its aponeurosis
Pictured here, we can see the internal oblique muscle with its aponeurosis connecting to the linea alba at the midline.

Origin & Insertion

The internal oblique originates from the thoracolumbar fascia, iliac crest, and the lateral two-thirds of the inguinal ligament. Its fibers run in a superomedial direction, and as they near the midline, they form an aponeurosis. This aponeurosis then inserts at the linea alba. The superior aspect of the internal oblique inserts onto the inferior borders of the tenth to twelfth ribs. Inferiorly, the internal oblique inserts at the pectineal line of the pubis (also known as the pecten pubis).[1,2,3]

origin of the internal oblique muscle on the inguinal ligament
Pictured here, we can see an origin of the internal oblique muscle on the lateral two-thirds of the inguinal ligament (highlighted in red).
origin of the internal oblique muscle on the iliac crest
Pictured here, we can see an origin of the internal oblique muscle on the iliac crest (highlighted in red).
origin of the internal oblique muscle on the thoracolumbar fascia
Pictured here, we can see an origin of the internal oblique muscle on the thoracolumbar fascia (highlighted in green).
internal oblique insertion on the inferior margins of tenth to twelfth rib
Pictured here, we can see an insertion of the internal oblique muscle on the inferior margins of the tenth to twelfth ribs (highlighted in blue). This is from a posterior view. From an anterior view, we would also be able to see it connecting to the adjacent costal cartilages.
insertion of the internal oblique muscle on the tenth rib and the adjacent costal cartilages
Pictured here, we can see the continuation of the insertion on the tenth rib, and we can also see the insertion connecting to the adjacent costal cartilages (highlighted in blue).
insertion of the internal oblique muscle on the linea alba
Pictured here, we can see an insertion of the internal oblique muscle on the linea alba highlighted in blue. The linea alba is a tendinous, fibrous line that runs vertically down the middle of the abdomen.
insertion of the internal oblique muscle on the pectineal line
Pictured here, we can see an insertion of the internal oblique muscle (highlighted in blue) on the pectineal line.

Action & Function

When acting with the external oblique, the internal oblique can cause flexion, rotation, and lateral flexion of the vertebral column. These movements depend on whether one side contracts (unilateral contraction) or both sides contract together (bilateral contraction). Unilateral contraction results in lateral flexion and rotation of the torso. Bilateral contraction results in flexion of the torso in an anterior direction. The internal oblique can also work with other abdominal muscles to increase intra-abdominal pressure.[1,3]

Image of a person demonstrating the movement of trunk flexion.
This image shows an example of trunk flexion, which involves bending the torso forward. The opposite of trunk flexion is trunk extension. Bilateral contraction of the internal oblique contributes to trunk flexion.
Image of a person demonstrating the movement of trunk lateral flexion.
This image demonstrates trunk lateral flexion, which involves bending the torso to the side. Unilateral contraction of the internal oblique contributes to trunk lateral flexion.
Image of a person demonstrating the movement of trunk rotation.
This image demonstrates trunk rotation, which involves twisting the waist/torso to the side. Unilateral contraction of the internal oblique contributes to trunk rotation.

Innervation

The internal oblique muscle is innervated by the anterior rami of the intercostal nerves (T7-T11), the subcostal nerve (T12), the iliohypogastric nerve (L1), and the ilioinguinal nerve (L1).[2,4]

intercostal nerves
Here, we can see the intercostal nerves highlighted in red. These nerves innervate the internal oblique muscle.
subcostal nerve
Here, we can see the subcostal nerve highlighted in red. The subcostal nerve innervates the internal oblique muscle.
iliohypogastric and ilioinguinal nerves
Here, we can see the iliohypogastric and ilioinguinal nerves labeled. These nerves innervate the internal oblique muscle. The iliohypogastric and ilioinguinal nerves originate from the lumbar plexus, with both nerves arising primarily from the anterior rami of the first lumbar nerve (L1). Sometimes, the iliohypogastric nerve also receives contributions from the twelfth thoracic nerve (T12). The iliohypogastric nerve has two branches, the lateral cutaneous branch and the anterior cutaneous branch. In this picture, the anterior branch is the one passing around to the front of the body.

Blood Supply

Blood is supplied to the internal oblique muscle via the lower posterior intercostal arteries and subcostal arteries, superior and inferior epigastric arteries, superficial and deep circumflex iliac arteries, and the posterior lumbar arteries.[2,4]

References

  1. [1] Flynn W, Vickerton P. Anatomy, Abdomen and Pelvis, Abdominal Wall. (Updated 2021 Jul 31). In: StatPearls (Internet). Treasure Island (FL): StatPearls Publishing; 2022 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK551649/
  2. [2] Varacallo M, Scharbach S, Al-Dhahir MA. Anatomy, Anterolateral Abdominal Wall Muscles. (Updated 2021 Jul 31). In: StatPearls (Internet). Treasure Island (FL): StatPearls Publishing; 2022 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK470334/
  3. [3] Moore KL, Agur AMR, Dalley AF. Clinically Oriented Anatomy. 8th ed. Philadelphia: Lippincott Williams & Wilkins; 2017
  4. [4] Seeras K, Qasawa RN, Ju R, et al. Anatomy, Abdomen and Pelvis, Anterolateral Abdominal Wall. (Updated 2021 Jul 26). In: StatPearls (Internet). Treasure Island (FL): StatPearls Publishing; 2022 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK525975/

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