The iliocostalis is a deep muscle of the back, one of the three muscles forming the erector spinae - the most powerful extensor of the back, the other two being the longissimus and spinalis muscles. The iliocostalis lies lateral to the longissimus. It has three parts according to their origin: iliocostalis cervicis, iliocostalis thoracis, and iliocostalis lumborum. The main function of the iliocostalis is to extend the spine when contracting on both sides, and to laterally flex the spine when contracting on one side.