Muscle:Internal Oblique
Internal oblique is the intermediate layer of the three lateral abdominal wall muscles. Its TrPs are less well characterised individually than the external oblique because the muscle is not directly accessible to flat palpation — central TrPs can only be reached by pincer grip of the lateral wall. Its pain patterns overlap those of the external oblique and transversus abdominis. TrPs in the lower internal oblique near the inguinal canal are a recognised source of bladder irritability, urinary frequency, urinary retention, and groin pain, and its fibres can entrap the ilioinguinal nerve where it pierces the muscle in a step-like or zig-zag fashion.
Anatomy
The internal oblique is a fan-shaped muscle. In the upright body its fibre direction ranges from nearly vertical posteriorly, through diagonally upward and medial among its intermediate fibres, to horizontal for the most caudal fibres. All fibres converge laterally onto the lateral half of the inguinal ligament, the anterior three-quarters of the iliac crest, and the lower portion of the lumbar aponeurosis.
- Above: Nearly vertical fibres attach to the cartilages of the last three or four ribs
- Diagonally: Fibres attach to the linea alba via the anterior and posterior rectus sheath
- Medially: Horizontal fibres from the inguinal ligament attach to the arch of the pubis through the conjoined tendon (formed jointly with the transversus abdominis)
Primary action: Increases intra-abdominal pressure (bilaterally); flexes the vertebral column (bilaterally); bends the vertebral column ipsilaterally (unilaterally); assists vertebral column rotation toward the contracting muscle (ipsilateral rotation), bringing the opposite shoulder forward. One external oblique and its contralateral internal oblique affect trunk rotation in the same direction.
Innervation: Branches of the eighth through twelfth intercostal nerves; branches of the iliohypogastric and ilioinguinal nerves from the first lumbar nerve. Segmental innervation T8–L1.
Memory aid for fibre direction: Place the left hand flat on the lower left abdomen — the fingers represent the internal oblique fibre direction on that side (and the direction of the intercostal muscles on the same side).
Referred Pain Patterns
Pain patterns of TrPs in this muscle are less consistent from patient to patient than for most other muscles, partly because the pain may come from any one of three successive layers and the muscle is difficult to isolate by palpation. Careful and thorough palpation is required to identify all potentially responsible TrPs.
Lower Internal Oblique — Bladder and Groin
TrPs in the lower internal oblique, along the upper rim of the pubis and the lateral half of the inguinal ligament, refer pain to:
- The urinary bladder region
- The groin
- The urethra
These TrPs increase irritability and spasm of the detrusor and urinary sphincter muscles, producing urinary frequency, urinary retention, groin pain, and — in older children — enuresis.
Costal Margin Attachment — Enthesitis
The region of attachment of the internal oblique to the costal margin in the region of the eleventh rib is vulnerable to developing enthesitis in response to overload. Each cough becomes excruciating. Latent TrPs in the muscle increase the likelihood of this development; continued coughing is a potent perpetuating factor.
Nerve Entrapment
The ilioinguinal nerve pierces the internal oblique and transversus abdominis muscles in a step-like or zig-zag fashion at a point approximately 3 cm medial to and slightly below the anterior superior iliac spine. TrP tension in the fibres of the internal oblique at this point can entrap the ilioinguinal nerve, producing referred pain to the iliac fossa, groin, and/or back.
When the entrapment is due to TrP activity in the fibres of the rectus abdominis or internal oblique, inactivation of the TrPs by injection with 0.5% procaine solution provides a simple way to relieve the symptoms. Neurolysis of the entrapped ilioinguinal nerve has been reported in cases where TrP treatment alone was insufficient.
Somatovisceral and Viscerosomatic Effects
Lower internal oblique TrPs can induce increased irritability and spasm of the detrusor and urinary sphincter muscles, causing urinary frequency, retention, and groin pain. In children, enuresis has been associated with these TrPs and reported to resolve after TrP injection.
Visceral disease can activate and perpetuate internal oblique TrPs, which may persist after the visceral disease has resolved — the same viscerosomatic cycle as for the external oblique.
Activation and Perpetuating Factors
Visceral Disease
Same as for the external oblique — peptic ulcer, intestinal parasites, dysentery, ulcerative colitis, diverticulitis, cholelithiasis.
Trauma and Surgery
Acute trauma and abdominal surgery; abdominal scar.
Postural and Mechanical
- Sustained twisted posture
- Over-enthusiastic abdominal exercise
- Paradoxical respiration
- Lateral bending — the internal obliques are more strongly activated than the external obliques during lateral bending movements
- Continued coughing (costal attachment enthesitis)
Other Factors
Emotional stress; cold exposure; structural inadequacies such as short leg or small hemipelvis.
Clinical Examination
The deep location of the internal oblique makes flat palpation unreliable for central TrPs. The preferred examination technique for central TrPs:
- Patient supine; flex hips to slacken the abdominal wall
- Grasp the lateral abdominal wall (flank area) between fingers and thumb in a pincer grip
- Roll the musculature between digits to identify tender nodules in palpable bands
Attachment TrPs near the pubic arch feel like small buttons or short bands at the region of attachment. The examiner must press down against the upper edge of the pubic arch — not on the flat anterior surface of the pubis.
The Abdominal Tension Test is performed as described for all abdominal muscles.
Differential Diagnosis
| Condition | Distinguishing features |
|---|---|
| Cystitis | Lower internal oblique TrPs produce urinary frequency, urgency, and bladder pain; referred TrP sensations have been diagnosed as cystitis; urinalysis and culture distinguish true cystitis |
| Prostatitis | Urinary tract symptoms from intrapelvic TrPs; exclude with digital rectal examination and PSA |
| Enuresis in children | Lower internal oblique TrPs associated with enuresis in older children; TrP injection has been reported to resolve the symptom |
| Inguinal hernia | Groin TrP pain without palpable hernia sac; Valsalva does not reproduce pain |
| Ilioinguinal neuralgia | Internal oblique nerve entrapment produces ilioinguinal distribution pain; distinguished from primary neuralgia by procaine injection at the TrP |
| Painful rib / slipping rib syndrome | Lower costal attachment enthesitis from internal oblique TrPs may contribute to presentations of slipping rib syndrome or rib-tip syndrome |
Treatment
Trigger Point Release
Because the internal oblique is not accessible to flat palpation for most central TrPs, injection using the pincer technique is the primary TrP intervention:
- Flex hips to slacken the abdominal wall
- Grasp the wall in a pincer grip between fingers and thumb, ensuring no abdominal contents remain within the grasp
- Direct the needle precisely into the TrP fixed within the operator's grasp
- Avoid penetrating the peritoneal cavity
Attachment TrPs along the costal margin and near the pubic arch respond to pressure release and injection as described for the external oblique.
Abdominal (diaphragmatic) breathing is the most effective active stretch exercise for the internal oblique. Selective activation of the internal oblique and transversus abdominis (the "drawing-in manoeuvre") is the basis of spinal stabilisation rehabilitation and appropriate corrective exercise.
Corrective Actions
- Abdominal (diaphragmatic) breathing, especially in the prone position
- Pelvic-tilt exercise (see Muscle:Rectus Abdominis — Corrective Actions)
- Self-administration of TrP pressure release
Satellite Trigger Points
- External oblique — primary functional partner; commonly co-active
- Transversus abdominis — deep synergist; forms conjoined tendon; commonly co-active
- Rectus abdominis — medial synergist
- Paraspinal muscles T8–L1 — key TrPs that activate internal oblique as satellite
- Iliopsoas — lower fibres near inguinal ligament; satellite relationship
Related Pages
- Pain:Groin and Testicle — Lower internal oblique referral pattern
- Pain:Urinary Frequency and Urgency — Internal oblique as myofascial source
- Pain:Abdominal — Diagnostic algorithm
- Muscle:External Oblique — Primary functional partner
- Muscle:Transversus Abdominis — Deep synergist and conjoined tendon partner
- Muscle:Rectus Abdominis — Medial synergist
References
- Travell JG, Simons DG. Myofascial Pain and Dysfunction: The Trigger Point Manual, Volume 2: The Lower Extremities. Baltimore: Williams & Wilkins; 1992. Chapter 49.