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Abdominal internal oblique muscle
Abdominal internal oblique muscle
Abdominal internal oblique muscle
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Abdominal internal oblique muscle
The abdominal internal oblique muscle.
Muscles of the trunk.
Details
OriginInguinal ligament, iliac crest and the lumbodorsal fascia
InsertionLinea alba, pectineal line of pubis (via conjoint tendon) and ribs 10-12.
ArterySubcostal arteries
NerveThoracoabdominal nn. (T7-T11), subcostal n. (T12), iliohypogastric n. (L1) and ilioinguinal n. (L1)
ActionsBilateral: Compresses abdomen Unilateral: Ipsilateral trunk rotation
Identifiers
Latinmusculus obliquus internus abdominis
TA98A04.5.01.017
TA22373
FMA13891
Anatomical terms of muscle

The abdominal internal oblique muscle, also internal oblique muscle or interior oblique or musculus obliquus abdominis internus, is an abdominal muscle in the abdominal wall that lies below the external oblique muscle and just above the transverse abdominal muscle.

Structure

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Its fibers run perpendicular to the external oblique muscle, beginning in the thoracolumbar fascia of the lower back, the anterior 2/3 of the iliac crest (upper part of hip bone) and the lateral half of the inguinal ligament. The muscle fibers run from these points superomedially (up and towards midline) to the muscle's insertions on the inferior borders of the 10th through 12th ribs and the linea alba.

In males, the cremaster muscle is also attached to the internal oblique.

Nerve supply

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The internal oblique is supplied by the lower intercostal nerves, as well as the iliohypogastric nerve and the ilioinguinal nerve.

Function

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The internal oblique performs two major functions. Firstly as an accessory muscle of respiration, it acts as an antagonist (opponent) to the diaphragm, helping to reduce the volume of the chest cavity during exhalation. When the diaphragm contracts, it pulls the lower wall of the chest cavity down, increasing the volume of the lungs which then fill with air. Conversely, when the internal obliques contract they compress the organs of the abdomen, pushing them up into the diaphragm which intrudes back into the chest cavity reducing the volume of the air-filled lungs, producing an exhalation.

Secondly, its contraction causes ipsilateral rotation and side-bending. It acts with the external oblique muscle of the opposite side to achieve this torsional movement of the trunk. For example, the right internal oblique and the left external oblique contract as the torso flexes and rotates to bring the left shoulder towards the right hip. For this reason, the internal obliques are referred to as "same-side rotators."

Additional images

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  • Diagram of a transverse section of the posterior abdominal wall, to show the disposition of the lumbodorsal fascia.
    Diagram of a transverse section of the posterior abdominal wall, to show the disposition of the lumbodorsal fascia.
  • Diagram of sheath of Rectus.
    Diagram of sheath of Rectus.
  • Diagram of a transverse section through the anterior abdomina wall, below the linea semicircularis.
    Diagram of a transverse section through the anterior abdomina wall, below the linea semicircularis.
  • Femoral sheath laid open to show its three compartments.
    Femoral sheath laid open to show its three compartments.
  • Lumbar triangle
    Lumbar triangle
  • Internal abdominal oblique muscle.Anterior abdominal wall.Deep dissection.Anterior view
    Internal abdominal oblique muscle.Anterior abdominal wall.Deep dissection.Anterior view

See also

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This article uses anatomical terminology.

References

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Revisions and contributorsEdit on WikipediaRead on Wikipedia

Abdominal internal oblique muscle

View on Grokipedia
from Grokipedia
The abdominal internal oblique muscle is a broad, sheet-like muscle forming part of the anterolateral abdominal wall, positioned deep to the external oblique muscle and superficial to the transversus abdominis muscle. It consists of thin, flat fibers that run superomedially, contributing to the layered structure that supports the and protects underlying viscera. This muscle originates from the anterior two-thirds of the , the , the iliopectineal arch, and the . Its fibers insert into the inferior borders of the 10th through 12th ribs, the linea alba, the pubic crest, and the pectin pubis via the , which it forms in conjunction with the transversus abdominis. These attachments enable the muscle to integrate with the and provide structural integrity to the . The internal oblique plays key roles in trunk movement and abdominal compression; bilateral contraction flexes the trunk and increases intra-abdominal pressure to aid expiration and visceral support, while unilateral contraction produces ipsilateral lateral flexion and rotation of the torso. It is innervated by the anterior rami of the lower thoracic nerves (T7–T12), including the thoracoabdominal, subcostal, iliohypogastric, and ilioinguinal nerves, which course between the internal oblique and transversus abdominis layers. Blood supply arises from the lower posterior intercostal and subcostal arteries, superior and inferior epigastric arteries, superficial and deep circumflex iliac arteries, and posterior lumbar arteries. Weakness or injury to this muscle can contribute to abdominal hernias by reducing wall tension.

Anatomy

Origin and insertion

The abdominal internal oblique muscle originates from the anterior two-thirds of the , the , the iliopectineal arch, and the lateral two-thirds of the . These origins position the muscle along the lateral abdominal wall, providing a broad base for its attachments. From these origins, the muscle fibers radiate in a fan-like manner, widening superiorly as they course toward their insertions. The posterior fibers insert into the inferior borders of the 10th, 11th, and 12th , while the anterior and intermediate fibers contribute to the linea alba through their and interdigitating inscriptions. Inferiorly, the muscle's fuses with that of the transversus abdominis to form the , which inserts into the pubic crest and pectineal line. This reinforces the posterior wall of the . The of the internal oblique muscle, in conjunction with that of the external oblique, forms the anterior layer of the .

Fiber orientation and

The abdominal internal oblique muscle exhibits fibers that run obliquely upward and forward in a superomedial direction, oriented at approximately 45 degrees to the midline and perpendicular to the inferomedial fibers of the superficial external oblique muscle. This crisscrossing arrangement with adjacent layers enhances the structural integrity of the anterolateral . The muscle spans from its origins on the , , and to insertions on the lower three or four and the linea alba via its . As the intermediate layer of the three flat abdominal muscles, the internal oblique lies deep to the external oblique and superficial to the transversus abdominis, forming a broad, thin sheet that contributes to the overall layering of the . Histologically, it consists of type I and type II fibers organized in a pennate-like , with occasional tendinous inscriptions that segment the muscle belly, aiding in force transmission. Its medially fuses with those of the external oblique and transversus abdominis; below the arcuate line, the aponeuroses of the internal oblique, external oblique, and transversus abdominis pass anterior to the rectus abdominis, contributing solely to the anterior layer of the . The muscle's thickness typically measures 5-10 mm, being thicker in lateral regions (mean approximately 8.5 mm) and tapering medially toward the aponeurotic portion. In males, the inferior fibers diverge to form the , which encircles the and contributes to testicular elevation.

Anatomical relations

The internal oblique muscle occupies the middle layer of the anterolateral , positioned deep to the external oblique and superficial to the transversus abdominis. Superficially, it is covered by the external oblique muscle and its throughout its extent. Deep to the internal oblique lies the transversus abdominis muscle, with the and parietal situated immediately beneath. Inferiorly, the muscle overlies the quadratus lumborum and forms the posterior wall of the , where its free inferior border arches over the alongside the transversus abdominis to create the . Medially, the aponeurosis of the internal oblique contributes to the formation of the , interdigitating with those of the external oblique and transversus abdominis; above the arcuate line, it splits into anterior and posterior layers enclosing the rectus abdominis, while below this line, it passes anteriorly to reinforce only the anterior sheath. Laterally, near its origins, the muscle lies adjacent to the iliacus and psoas major muscles via the iliopsoas fascia and lateral . Intercostal nerves and vessels course in the neurovascular plane between the internal oblique and the deeper transversus abdominis.

Blood supply

The arterial supply to the abdominal internal oblique muscle primarily derives from the lower posterior intercostal and subcostal arteries, superior and inferior epigastric arteries, superficial and deep circumflex iliac arteries, and posterior lumbar arteries. The lower five posterior intercostal arteries (seventh through eleventh), which originate from the thoracic aorta, provide nourishment to the upper and middle portions of the muscle. The subcostal artery, arising from the thoracic aorta at the level of the twelfth thoracic vertebra, contributes to the supply in the lower thoracic region. Inferiorly, the deep circumflex iliac artery, a branch of the external iliac artery, ascends along the iliac crest to vascularize the lower third of the muscle via its ascending branches. These vessels collectively ensure robust perfusion, supporting the muscle's role in abdominal wall dynamics. Venous drainage of the internal oblique muscle parallels its arterial supply, with corresponding s accompanying the intercostal and subcostal arteries to drain superiorly into the azygos and hemiazygos veins via the posterior intercostal veins. Inferiorly, tributaries from the deep circumflex iliac converge to form the , facilitating efficient return of deoxygenated blood from the lower muscle segments. This symmetric vascular architecture minimizes stasis and maintains hemodynamic balance across the muscle's extent. Lymphatic drainage follows the vascular pathways, with superior lymphatics accompanying the intercostal vessels to and inferior channels draining via the deep circumflex iliac pathway to nodes. This drainage pattern integrates the muscle into the broader , aiding in fluid and immune surveillance. The neurovascular bundles, including these vessels, penetrate the internal oblique muscle from its deep surface and course within the plane between the internal oblique and transversus abdominis layers, branching to supply the muscle fibers without disrupting the superficial .

Innervation

The abdominal internal oblique muscle is innervated by the anterior rami of the lower six thoracic spinal nerves (T7–T12), collectively referred to as the thoracoabdominal nerves, which provide the primary motor supply to its fibers. The subcostal nerve, arising from T12, contributes to the innervation of the lower portion of the muscle, while the iliohypogastric and ilioinguinal nerves, both originating from the L1 spinal nerve, supply the most inferior aspects near the inguinal region. These nerves deliver segmental motor innervation, enabling coordinated contraction of the muscle during activities involving trunk stabilization and movement. In addition to motor functions, the same thoracoabdominal nerves (T7–T12), along with the iliohypogastric and ilioinguinal nerves, provide sensory innervation to the skin of the anterolateral abdominal wall. For instance, the thoracoabdominal nerves convey cutaneous sensation from dermatomes overlying the abdomen, with T10 corresponding to the region around the umbilicus, while the iliohypogastric nerve supplies sensation to the skin over the suprapubic area and upper buttocks, and the ilioinguinal nerve innervates the skin near the inguinal ligament and medial thigh. The nerves approach the internal oblique muscle from its deep surface, having first pierced the underlying transversus abdominis muscle to enter the neurovascular plane between these two layers. This pathway ensures efficient distribution of both motor and sensory branches throughout the muscle. Innervation of the internal oblique muscle is bilateral and independent on each side, allowing for unilateral activation that supports asymmetric actions such as lateral flexion or rotation of the trunk without contralateral involvement.

Function

Primary actions

The abdominal internal oblique muscles, upon bilateral contraction, compress the abdominal viscera, flex the trunk forward, and increase intra-abdominal . These actions support core integrity and facilitate physiological processes requiring elevated within the abdomen. Unilateral contraction of the internal oblique produces lateral flexion (side-bending) of the trunk toward the same side and ipsilateral of the trunk. The muscle's superomedial fiber orientation contributes to these rotational . In coordination with the contralateral external oblique muscle, the internal oblique facilitates of the trunk toward the same side; for example, contraction of the right internal oblique aids in rightward trunk . Through force generation via contraction, the internal oblique contributes to spinal stabilization during dynamic activities such as coughing or lifting, where it helps maintain vertebral column posture and supports abdominal contents against gravitational and exertional demands.

Role in respiration and stability

The internal oblique muscle functions as an accessory expiratory muscle, contributing to forced by contracting to elevate intra-abdominal pressure and displace the diaphragm cephalad, thereby reducing thoracic volume and expelling air from the lungs. This action becomes prominent during activities requiring increased ventilatory effort, such as exercise or coughing, where it synergizes with other muscles to augment alveolar pressure and facilitate active expiration beyond passive recoil. Unlike quiet , which relies primarily on the diaphragm, the internal oblique's involvement ensures efficient clearance of air during high-demand scenarios, supporting overall respiratory regulation. In , the internal oblique plays a key role in generating and maintaining intra-abdominal (IAP), which unloads compressive forces on the spine by creating an extension moment that counters flexion from trunk muscles. This mechanism enhances spinal stability, with studies showing that doubling IAP from 5 kPa to 10 kPa can increase stability by a factor of approximately 1.8 during various efforts, such as axial or extension. The muscle integrates with the diaphragm superiorly and the inferiorly to form a functional pressure cylinder, where coordinated activation distributes forces evenly to support the spine and viscera without excessive strain. The internal oblique synergizes with the transversus abdominis to regulate IAP, as both muscles exhibit co-activation patterns that enhance trunk stabilization, with methods like multisensory cuing during exercises increasing their for improved function. This interaction is evident in postural control, where bilateral contraction of the internal oblique elevates the anterior pelvis, countering anterior and promoting balance during standing or by maintaining abdominal tension and trunk alignment. Such contributions underscore its role in preventing postural deviations and supporting dynamic equilibrium.

Clinical significance

Injuries and conditions

Strains of the abdominal internal oblique muscle commonly occur in sports involving sudden twisting or rotational movements, such as , , and , often at the musculotendinous junction near the lower or . These injuries result from eccentric overload during forceful contractions, leading to partial or complete tears. Symptoms typically include acute, unilateral pain in the lateral exacerbated by movement, localized tenderness upon , and reduced strength or pain during resisted trunk , reflecting the muscle's role in ipsilateral . Weakness or tears in the internal oblique muscle can contribute to direct inguinal hernias by compromising the structural integrity of the inguinal canal's posterior wall. Hematomas of the internal oblique muscle may develop following direct trauma, such as blunt abdominal , or even non-traumatic events like severe coughing, presenting with sudden severe , bruising, and a firm palpable mass along the lateral . relies on clinical evaluation, including history of inciting trauma or activity and physical tests such as resisted side-bending or rotation to reproduce pain, confirmed by imaging modalities like for acute hematomas or MRI to detect muscle tears and . Risk factors for internal oblique injuries include repetitive core-loading activities in athletes, , and , which elevate intra-abdominal pressure and strain the muscle. Incidence is notably higher among athletes, with oblique muscle injuries being among the most common injuries in rotational sports like and soccer, where injuries can account for up to 7% of all injuries; as of 2024, abdominal oblique and rectus muscle injuries represent the third most prevalent time-loss injury in . Recurrence rates reach up to 12% in players.

Surgical relevance

The abdominal internal oblique muscle is integral to repair due to its formation of the , a key structure for reinforcing the posterior wall of the . In the Bassini technique, the —comprising the aponeuroses of the internal oblique and transversus abdominis—is sutured to the to close the defect and prevent recurrence. The McVay repair similarly utilizes the , suturing it to Cooper's ligament while incorporating a relaxing incision in the internal oblique to facilitate tension-free closure, particularly for larger defects. In laparoscopic approaches like the totally extraperitoneal (TEP) method, dissection for deep inguinal ring access occurs adjacent to the internal oblique, which contributes to the canal's roof and posterior wall, allowing placement without direct muscle splitting. In abdominoplasty procedures addressing diastasis recti, plication of the anterior tightens the overlying , including contributions from the internal oblique muscle, to restore integrity and reduce midline separation. This suture-based reinforcement, often using permanent materials along the linea alba, corrects the widened inter-rectus distance typically exceeding 2 cm, enhancing without excising muscle tissue. Other surgical interventions frequently traverse the internal oblique layer, such as in open appendectomy via the McBurney incision, where the muscle fibers are split along their direction following external oblique aponeurotomy to access the . Similarly, in cesarean delivery using the Pfannenstiel approach, the anterior incision incorporates the fused aponeuroses of the internal oblique and transversus abdominis, though the muscle bellies are retracted rather than divided. During ilioinguinal , often performed adjunctively in repairs, the nerve's course between the external and internal oblique layers poses a risk of iatrogenic , potentially leading to chronic groin pain if not preserved or carefully mobilized. Postoperative management emphasizes preserving the internal oblique's innervation from branches of the lower intercostal and subcostal nerves to prevent weakness or bulge formation. In ventral repairs, synthetic mesh is commonly placed as an onlay over the internal oblique or in a sublay position posterior to it, promoting tissue ingrowth and reducing recurrence rates to under 10% in select cases.

Development and variations

Embryological development

The abdominal internal oblique muscle originates from myoblasts derived primarily from the hypaxial domain of the myotomes in somites T7 to L1, which form part of the , during weeks 4 to 6 of (approximately 14 to 16). These myoblasts arise from the dorsolateral portion of the somites and contribute to the skeletal musculature of the ventral body wall, while associated connective tissues derive from the somatic layer of the (somatopleure). During subsequent development, the myoblasts differentiate into muscle fibers and migrate ventrally from their dorsal origins, integrating into the forming layers by week 8 (around Carnegie stage 23). This migration occurs as the common myogenic mass splits, with the internal oblique becoming partially distinct from the transversus abdominis by stage 19 (week 7), facilitated by that guide separation and orientation. The fibers then fuse with developing aponeuroses, forming the intermediate layer of the anterolateral abdominal wall and contributing to the linea alba as the structures cross the midline. In males, a specialized component of the internal oblique differentiates into the , originating from during testicular descent between weeks 7 and 9; fibers from the internal oblique loop around the , forming the cremasteric layer as the testis migrates through the . By week 12, the muscle achieves full segmental patterning, influenced by expression in the presomitic and somitic , which establishes anterior-posterior identity and ensures proper myotomal specification for thoracic and levels.

Anatomical variations

The abdominal internal oblique muscle exhibits several anatomical variations, primarily in its attachments, thickness, and associations with adjacent structures. Common variations include asymmetrical fiber thickness, with studies in healthy individuals reporting mean of 11% to 25% for the internal oblique relative to the contralateral side, reflecting natural bilateral differences in muscle development. Another frequent variation involves the , formed by the aponeurotic contributions of the internal oblique's lower fibers and those of the transversus abdominis; or absence of this tendon has been documented, potentially weakening the posterior wall of the . Rare anomalies of the internal oblique include complete fusion with the transversus abdominis muscle, a condition noted in historical anatomical dissections that complicates surgical or cadaveric separation of the layers. Accessory slips may also arise, such as a thickened portion from the dividing into anterior and posterior segments, forming an accessory internal oblique that extends to the iliac fascia. Gender differences manifest in the attachment, which originates as slips from the internal oblique and is more extensively developed in males to encircle the , whereas in females it is rudimentary along the round . During , the thickness of the internal oblique muscle typically decreases, with recovery taking longer than 6 months postpartum. Such deviations can increase risk by reducing stability.

References

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