The internal oblique is located directly under the external oblique, and at its upper end its fibers run at nearly right angles to the external oblique fibers, forming an inverted letter V when viewed from the front. At the lower end, however, the internal oblique fibers are almost horizontal. Both the internal and external obliques cover a very wide area on the front side of the abdomen.
The internal oblique muscle originates on the lumbar fascia, the anterior crest of the ilium, and the outer half of the inguinal ligament.
At the upper end it inserts on the cartilage of the eight, ninth, and tenth ribs and the linea alba.
The internal oblique muscles are responsible for rotation of the upper and lower trunk, lateral flexion, and flexion of the spine.