The internal oblique is an abdominal muscle located beneath the external abdominal oblique.
This muscle originates at the lumbar fascia (a connective tissue that covers the lower back), the outer portion of the inguinal ligament (a ligament located on the bottom-outer edge of the pelvis), and back of the iliac crest (the upper-outside portion of the pelvis). The internal abdominal oblique muscle ends at the bottom edge of the rib cage, the rectus sheath (fibrous tissue that covers the abdominal muscles), and the pubic crest (an area in the lower-front of the pelvis).
The internal abdominal oblique muscle is located closer to the skin than the transverse abdominal muscle.
This muscle supports the abdominal wall, assists in forced respiration, aids in raising pressure in the abdominal area, and rotates and turns the trunk with help from other muscles.
The internal abdominal oblique muscle is an opposing force to the diaphragm, reducing upper chest cavity volume during exhalation. As the diaphragm contracts, the chest cavity is pulled down to increase lung size.
The contraction of this muscle also rotates the trunk and bends it sideways 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.