The internal oblique is an abdominal muscle located beneath the external abdominal oblique. This muscle originates at the lumbar fascia, the lateral portion of the inguinal ligament and anterior portion of the iliac crest. The internal abdominal oblique muscle is superficial to the transverse abdominal muscle. The internal abdominal oblique muscle inserts at the costal margin, the aponeurosis of the rectus sheath and the pubic crest. This muscle supports the abdominal wall, assists in forced respiration, aids in raising the intra-abdominal pressure, and rotates and abducts the trunk with help from other muscles. The internal abdominal oblique muscle is an antagonist to the diaphragm, reducing thoracic chest cavity volume during exhalation. As the diaphragm contracts, the chest cavity is pulled down to increase lung volume. The contraction of this muscle also side-bends and rotates the trunk by pulling the midline and rib cage toward the lower back and hip. Internal abdominal oblique muscles are called same side rotators. The right internal oblique works with the left external oblique and vice versa when flexing and rotating the torso.
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In Depth: Internal oblique