INTERNAL OBLIQUE MUSCLE: ORIGIN, IRRIGATION, FUNCTIONS, PATHOLOGIES - ANATOMY AND PHYSIOLOGY - 2025

The internal oblique or minor oblique muscle, as it is also known, is part of the muscles of the anterolateral area of ​​the abdomen. Its name comes from the Latin musculus obliquus internus abdominis. It is a wide muscle, with a flattened appearance and depending on its origin and insertion it can be said that it joins the torso with the pelvis.

The internal oblique, together with the external oblique muscle and the transverse abdominal muscle make up the muscle group called broad abdominal muscles, that is, they make up the anterolateral abdominal wall. It is also part of the expiratory muscles.

Graphic representation of the location of the internal oblique muscle. Source: Henry Vandyke Carter. Edited image

The internal oblique muscle is located below the external or greater oblique and above the transverse abdominis muscle. This muscle is considered to be the continuation of the internal intercostals.

It is a pair muscle that is symmetrically located one on each side of the body. The fibers located laterally are fleshy and those located medially are aponeurotic. Its fibers project obliquely and upward (superolateral).

The muscle is covered by a membrane called the aponeurosis in its anterior part that reaches the linea alba. Its free lower border adjoins the anterior part of the deep inguinal ring.

The main function of this muscle is to provide support and protection to the internal organs of the abdomen, since together with the rest of the anterolateral muscles of the abdomen they form a resistant three-layer girdle.

Second, it flexes and rotates the trunk, and thirdly, it helps in expiratory movements and abdominal contractions necessary during urination, defecation, and childbirth.

Origin

The muscle fibers of the internal oblique of the abdomen exit from the iliac crest through its external border, collaborating in the formation of the inguinal ligament in its medial part and the lumbar fascia in its entirety.

Insertion

The muscle fibers are inserted in the cartilage of the last ribs, ranging from the crest of the pubis, passing through the pectineal line and the linea alba at the level of the lumbar spine. It joins the pectineal line thanks to the transverse aponeurosis.

Innervation

The internal oblique muscle reaches the lower intercostal nerves from T7-to T12, the lesser or ilioinguinal genital abdomen and branches of the iliohypogastric nerve or greater genital abdomen.

Irrigation

The internal oblique muscle is supplied by the subcostal arteries.

Features

It is an expiratory muscle, because during the release of air it compresses the chest at its lower end while the pelvis and spine remain fixed.

In this sense, it works in conjunction with the rest of the expiratory muscles, which are mentioned below: internal intercostal muscles, external oblique, levator ani, triangular sternum, transverse, pyramidal and rectus abdominis.

The layered structural design that makes up the anterolateral abdominal muscles, including the internal oblique, forms a strong girdle that supports and protects internal organs.

Graphic representation of the 3 layers that make up the anterior abdominal wall. Source: Dr. Johannes Sobotta. Edited image.

When the internal oblique muscle contracts in the company of its counterpart, they can flex the spine, while when they act separately they are able to tilt the trunk to one side or the other, depending on the muscle that is active. They can also rotate the chest.

On the other hand, this muscle plays a very important role in maintaining intra-abdominal pressure when urinating, defecating, and giving birth.

Pathologies

- Tear of the internal oblique muscle

The internal oblique muscle can be injured in athletes who require torso rotation. The injuries are caused by excessive contraction of an eccentric and disproportionate nature.

Maquirriain et al. Described several clinical cases in which the internal oblique muscle was injured.

The first case was a 22-year-old tennis player who, during an uncontrolled rotational maneuver, suffered an injury that caused a lot of pain in the anterolateral wall. MRI revealed a first degree tear of the left internal oblique muscle.

The second case reported was that of a 32-year-old professional runner. During an obstacle course he tripped over one of the fences, which caused a sudden rotation of the trunk and subsequent fall.

The athlete presented a lot of pain below rib 12 on palpation and pain when trying to flex or rotate the trunk. Magnetic resonance imaging revealed an intrasubstantial tear of the right internal oblique muscle.

Fortunately, recovery from the reported tears was quick. The patients followed a conservative treatment, represented by rest, cryotherapy and administration of oral non-steroidal anti-inflammatory drugs, with subsequent strengthening of the muscles with physical exercises.

- Abdominal diastasis

Understand the separation of the abdominal muscles from the midline. This involvement is caused by damage to the joint tissue. It can happen after pregnancy or in very obese people.

This condition occurs due to the weakening of the anterolateral muscles of the abdomen, including the internal oblique and the covering aponeuroses. The separation creates a free space that is filled by the abdominal fascia, which is a much thinner layer.

The clinical manifestation is a flaccid-appearing abdomen and low back pain.

Abdominal diastasis can be corrected with exercises and physical therapy, but when the separation is greater than 5 cm, the only way to correct it is by surgery.

Graphic representation of diastasis. Source: Anež & Han. Edited image.

- Inguinal and lumbar hernia

Inguinal and lumbar hernias are the product of soft tissues or viscera exiting through a hole caused by the separation or rupture of some weak muscle tissue in the abdominal wall. They are produced by sudden and unexpected physical efforts.

The internal oblique muscle may be involved in the formation of the hernia. The most common hernias are inguinal hernia and lumbar hernia.

Inguinal hernia

A vulnerable site for herniation is the inguinal canal. On the roof of the inguinal canal is the internal oblique muscle and the transverse abdominis.

It should be noted that most of the time inguinal hernias are resolved with surgery. In this sense, a crucial anatomical site for surgical intervention is that of the junction of the transverse aponeurosis with that of the internal oblique, at the level of the rectum sheath.

Lumbar hernia

Petit's hernia originates in the lower lumbar triangle. This hernia is caused by the weakening of the internal oblique muscle.

Grynfeltt's hernia originating in the superior lumbar triangle can also be mentioned. This triangle is bounded on its anterior side by the internal oblique muscle.

Graphic representation of a hernia. Source: File: Umbilical hernia growth.gif. Edited image.

References

1. Maquirriain J, Ghisi J, Megey J, Mazzuco J. Abdominal internal oblique muscle injuries in athletes. Arthroscopy, 2003; 10 (1): 1-3. Available at: revistaartroscopia.com
2. «Abdominal internal oblique muscle» Wikipedia, The Free Encyclopedia. 24 Nov 2019, 01:16 UTC. 28 Dec 2019, 15:40 en.wikipedia.org/
3. López P, López F, Puentes E, González O, Ochoa F, Cruz N, et al. A new surgical approach performed in inguinal herniorrhaphy. Rev Cubana Cir. 2004; 43 (2). Available at: scielo.sld
4. Gac P, Uherek F, Del Pozo M, Oropesa A and Rocco E. Inguinal hernia: a permanent surgical challenge. Notebooks of Surgery, 2011; 15 (1): 96-106. Available at: magazines.uach.cl/
5. Saldaña E. (2015). Manual of human anatomy. Available at: oncouasd.files.wordpress