The iliocostalis is a muscle of the back included in the erector spinae muscle group. It is most lateral of the erector spinae muscles, in that it is the farthest from the spine, and it vertically spans the cervical, thoracic, and lumbar vertebrae. A very long, very narrow muscle, it is responsible for spinal movements involving both the head and neck and the entire vertebral column.
As part of the erector spinae group, which is actually three distinct muscles arranged in columns, the iliocostalis is accompanied by spinalis and longissimus. The spinalis is located medially or nearest to the spine and consists of three segments stretching from the upper two lumbar vertebrae up to the second cervical vertebrae: the spinalis dorsi, the spinalis cervicis, and the spinalis capitus. Between the spinalis and the iliocostalis is the longissimus, which is the longest of the three and which also consists of three segments: the longissimus dorsi, the longissimus cervicis, and the longissimus capitus.
To the outside of these two muscles is the iliocostalis with its three segments: the iliocostalis lumborum, the iliocostalis dorsi, and the iliocostalis cervicis. The segments of the iliocostalis are staggered, with the iliocostalis lumborum segment originating at the base of the spine from the sacrum and the crest of the ilium bone in the hip, a point of origin it shares with the rest of the erector spinae muscles. From there it ascends to insert via tendons to the lower six or seven ribs.
The iliocostalis dorsi arises from the lower six ribs and runs vertically just medially or to the inside of the fibers of the iliocostalis lumborum. It inserts on the upper six ribs as well as on the transverse processes of C7, the small bony prongs on the front side of the seventh cervical vertebra. Finally, the iliocostalis cervicis originates on the third through sixth ribs and attaches to the transverse processes of the fourth through sixth cervical vertebrae in the neck.
It is in the neck where one of the major actions of the iliocostalis can be seen. When the muscle contracts laterally, or on one side only, it flexes the head and neck toward the side on which it is contracting. When the muscle contracts bilaterally — that is, when both sides are activated at once — it assists in spinal extension, or the straightening-up of the vertebral column, as in drawing oneself upright from a hunched-over position.