The iliacus muscle is shaped like a triangle, flat and an exact fit of the iliac fossa — the curved surface of the largest pelvic bone. Together with the psoas major muscle, it is also called the iliopsoas muscle.A portion of this muscle is attached to the iliac fossa, two-thirds from its top. Another portion is attached to the inside portion of the iliac crest, the top, outer portion of the pelvic bone.Other fibers of this muscle are attached to the iliolumbar and anterior sacroiliac ligaments (located at the base portion of the sacrum) and up to the anterior iliac spines (bony projections that lie toward the edges of the iliac). These muscle fibers then converge and insert on the tendon at the lateral (outer) side of the psoas major muscle, which stretches from the lumbar spine in the lower back to the lower pelvis. Some of these fibers extend to the femur bone, or thighbone.[1]