(il'ē-ō-kos-tā'lis lum-bō'rŭm mŭs'ĕl) Origin, with erector spinae; insertion, the angles of lower six ribs; action, extends, abducts, and rotates lumbar vertebrae; nerve supply, dorsal branches of thoracic and lumbar nerves. Synonym(s): lumbar iliocostal muscle.