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Anatomy of The Lumbar Back Muscles-Internal Oblique

The internal oblique is the largest muscle of the abdominal wall . The internal oblique forms the middle layer of the abdominal wall between the TrA and the external oblique. It originates from the “lateral two-thirds of the inguinal ligament, anterior two-thirds of the iliac crest, and the lateral raphe of the thoracolumbar fascia in a band 2–3 cm wide, attaching to fibers of the deep lamina arising from the L3 spinous process” .

The posterior layer of the fascia passes posterior to the rectus abdominis and is continuous with the TrA, whereas the anterior fibers are anterior to the rectus abdominus and continuous with the contralateral external oblique. The internal oblique has been recognized as a global muscle, although some portions appear to function in the local support system, according to Bergmark, because of the posterior fibers attaching to the lateral raphe.

The lower anterior fibers compress and work with the TrA to support the lower abdominal viscera. The upper anterior and lateral fibers, when acting bilaterally, flex the vertebral column and assist in respiration. When acting unilaterally, along with the external oblique on the opposite side, rotate the vertebral column and bring the thorax backward or pelvis forward.

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