any of the deep muscles of the back that, as part of the erector spinae (sacrospinalis) muscle group, aid in extension (bending backward), lateral flexion (bending to the side), and rotation of the spinal column. The iliocostalis group consists of a lower part (iliocostalis lumborum) that extends from the ilium (upper part of the hipbone) to the lower ribs, a middle part (iliocostalis dorsi, or thoracis) that extends from the lower to the upper ribs, and an upper part (iliocostalis cervicis) that extends from the upper ribs to the transverse processes (side projections) of the cervical vertebrae (in the neck).