The iliacus muscle is found in the lower portion of the trunk, covered in a thick fascia (connective tissue). This 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.
The iliacus muscle is supplied with nerves by the branches of the third and second nerves of the lumbar area through the femoral nerve.
The iliacus is part of the hip rotator muscles, which are responsible for the flexing of the thigh on the pelvis and the forward tilting of the pelvis. It is also one of the key muscles that helps to maintain proper body posture.