The internal oblique works with the contralateral external oblique to rotate the rib cage down and toward the pelvis, flattening the ipsilateral lumbar spine, while pulling the pelvis anteriorly into counternutation.

Origin: Iliac crest, inguinal ligament and thoracolumbar fascia

Insertion: Lower 1/3 inserts into the aponeurosis of the linea alba

• The upper 2/3 splits to go around the rectus abdominis to attach to the linea alba.
• The posterior fibers of the upper 2/3 blend with the aponeurosis of the transverse abdominis to attach to the linea alba.
• The anterior fibers blend with the aponeurosis of the contralateral external oblique to attach to the linea alba.
• The upper fibers also attach to the costal cartilages 7 to 9, and to the lower 3 or 4 ribs [1] ^p521-522.
  

Functions:

• Controls abdominal contents, increasing internal abdominal pressure (IAP) [2]^p1064
• Bilateral contraction causes trunk flexion by approximating the thorax to the pelvis [1]^p521-527
•  Ipsilateral contraction causes ipsilateral trunk rotation and ipsilateral lumbar flexion (flattening) [1] ^p521-527, bringing the rib cage towards the ipsilateral pelvis; in this action, it coordinates with the contralateral external oblique.
• Helps stabilize the pelvis during leg movement [3]
• Observation reveals that it pulls the ipsilateral ilium anteriorly and superiorly.

In sacroiliac joint lesions, the internal oblique was found to be hypertonic [4-6] and exhibit 25% less strength [7].

When measured by EMG, Snijders [5] found that standing on the contralateral leg significantly reduced internal oblique activity ipsilateral to the lesioned sacroiliac joint. This movement took pressure off the ipsilateral sacroiliac joint. It appeared that the internal oblique was hypertonic to help stabilize the sacroiliac joint. As the pressure was removed from the sacroiliac joint, the activity of the internal oblique decreased.

In a subsequent test, Snijders [5] found that wearing a pelvic belt significantly decreased EMG activity in the internal oblique. A pelvic belt may be interpreted as a stabilizing force on the sacroiliac joint. With the belt on, less force is needed from the internal oblique to stabilize the sacroiliac joint against gravitational force, indicating that the internal oblique provides a counternutation response to the nutation effects of gravity.

Snijders [4] and Vleeming[6] both found that the internal oblique was more active in sitting and standing (activities that induce nutation), than in the supine position (which induces counternutation). It appeared that, in sitting and standing, the nutation lesion was stressed and the internal oblique was activated to counter the lesion. They also found that the external oblique and, to a greater extent, the internal oblique, both potential counternutators when acting on the side of the internal oblique, were significantly active during erect standing (resisting gravity), while the gluteus maximus, erector spinae, and biceps femoris, all potential nutators, were relatively inactive.

Snijders [8] found that, in forward bending, the additional stress placed on the sacroiliac joint can be countered by the oblique abdominal muscles, as well as a sacroiliac belt, indicating that the obliques provided a stabilizing force to the sacroiliac joint during forward bending, which is a mechanism of injury in the nutation lesion.

Hemborg [7] demonstrated that the internal oblique has 25% less strength in low back pain patients than in controls. Combined with the above studies that show the internal oblique was more active in patients with sacroiliac joint pain and less active when the sacroiliac joint was stabilized by a sacroiliac belt, it can be assumed that stabilization to the sacroiliac joint can cause increased strength in the internal oblique and, possibly, other muscles that contract to stabilize the sacroiliac joint.

Sacroiliac Joint: [9] demonstrated that delayed feed forward activation in the transverse abdominis and internal oblique muscles was significantly improved by sacroiliac manipulation, demonstrating a definite relationship between the sacroiliac joint and muscular reflex timing patterns.

While standing erect, internal oblique muscle activity significantly decreased while wearing a sacroiliac belt, which indicates that a pelvic belt contributes to SIJ stability. Snijders et al. [5]

References:

1. Warwick, R. and P.L. Williams, eds. Gray’s Anatomy. 35 ed., ed. R. Warwick and P.L. Williams. 1973, W.B. Saunders Company: New York.
2. Standring, S., et al., eds. Gray’s Anatomy. 40th ed. 2008, Churchill Livingstone: London.
3. Floyd, W.F. and P.H. Silver, Electromyographic study of patterns of activity of the anterior abdominal wall muscles in man. J Anat, 1950. 84(2): p. 132-45.
4. Snijders, C.J., et al., Oblique abdominal muscle activity in standing and in sitting on hard and soft seats. Clinical Biomechanics (Bristol, Avon), 1995. 10(2): p. 73-78.
5. Snijders, C.J., et al., EMG recordings of abdominal and back muscles in various standing postures: validation of a biomechanical model on sacroiliac joint stability. Journal of Electromyography and Kinesiology, 1998. 8(4): p. 205-14.
6. Vleeming, A., et al., The role of the sacroiliac joints in coupling between spine, pelvis, legs and arms., in Movement, Stability, and Low Back Pain, A. Vleeming, et al., Editors. 1997, Churchill Livingstone. p. 53-71.
7.  Hemborg, B. and U. Moritz, Intra-abdominal pressure and trunk muscle activity during lifting. II. Chronic low-back patients. Scandinavian Journal of Rehabilitation Medicine, 1985. 17(1): p. 5-13.
8. Snijders, C.J., Transfer of Lumbosacral Load to Iliac Bones and Legs: Part 2 – Loading of the Sacroiliac Joints when Lifting in a Stooped Position. Clinical Biomechanics, 1993b. 8: p. 295-301.
9. Marshall, P. and B. Murphy, The effect of sacroiliac joint manipulation on feed-forward activation times of the deep abdominal musculature. Journal of Manipulative and Physiological Therapeutics, 2006. 29(3): p. 196-202.