The internal obliques attach from the lower three ribs to the linea alba and from the the inguinal ligament to the iliac crest and then to the the lower back (erector spinae). The lower muscle fibers of the internal obliques run nearly horizontally.
Along with the external obliques, the internal obliques are involved in flexing the spinal column, sideways bending, trunk rotation and compressing the abdomen.
Because of their unique alignment, at right angles to each other, the internal and external obliques are referred to as opposite-side rotators. When the trunk rotates left, the external obliques (on the right) contract. When the trunk rotates to the right, the external oblique fibers (on the left) activate the movement.