The iliacus is a large triangular muscle that lies along the lateral side of the inferior part of the psoas major. Most of its fibers join the tendon of the psoas major. Together the psoas and iliacus form the iliopsoas, the chief flexor of the thigh. It is also a stabilizer of the hip joint and helps maintain the erect posture at this joint. The psoas and iliacus share in hip flexion; however, only the psoas can produce movement (flexion or lateral bending) of the lumbar vertebral column.