The internal oblique is the intermediate muscle of the abdomen, lying just underneath the external oblique and just above (superficial to) the transverse abdominal muscle. It's fibers run perpendicular to the external oblique, 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 superiomedially (up and towards midline) to the muscle's insertions on the inferior borders of the 10th thru 12th ribs and the linea alba (abdominal midline seam.) Human male pelvis, viewed from front Human female pelvis, viewed from front The pelvis is the bony structure located at the base of the spine (properly known as the caudal end). ... The inguinal ligament is a band running from the pubic tubercle to the anterior superior iliac spine. ... The linea alba is a fibrous structure that runs down the midline of the abdomen in humans and other animals. ...
It performs two major functions. Firstly, it acts as an antagonist (opponent) to the diaphragm, helping to reduce the volume of the thoracic (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. A diaphragm is some sort of separating membrane. ... A diaphragm is some sort of separating membrane. ...
Secondly, its contraction rotates and side-bends the trunk by pulling the rib cage and midline towards the hip and lower back, of the same side. It acts with the external oblique of the opposite side to achieve this torsional movement of the truck. 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."
References
Moore, Keith L; & Dalley Arthur R (2006). Clinically Oriented Anatomy (5th ed.). Lippincott Williams and Wilkins. ISBN 0781736390.
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 internaloblique is innervated by the lower intercostal nerves, as well as the iliohypogastric nerve and the ilioinguinal nerve.
For example, the right internaloblique and the left external oblique contract as the torso flexes and rotates to bring the left shoulder towards the right hip.
Inferiorly, the medial portion of the fused internaloblique and transversus aponeuroses is termed the conjoined tendon.
The fascia on the internal surface of the transversus abdominis serves as epimysium and is known as the transversalis fascia.
The anterior sheath is comprised of the aponeurosis of the external oblique and an anterior layer of the aponeurosis of internaloblique; the posterior sheath of the posterior layer of the internaloblique and the aponeurosis of the transversus abdominis muscle.
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