Two sets of muscles are involved in extension of the spine: a deep spinal group and a superficial spinal group. The superficial group is collectively known as the erector spinae muscle group, which consists of four separate but intertwined muscles. They are the iliocostalis thoracis, the iliocostalis lumborum, the longissimus dorsi and the spinalis dorsi.
The erector spinae muscle group originates on the crest of the ilium, the lower surface of the sacrum, the borders of the lower seven ribs, the spinous processes of all the lumbar and the lower four thoracic vertebrae, and the transverse processes of all the thoracic vertebrae.
Muscle insertion is on the angles of the ribs and on the transverse processes of all vertebrae.
Extend and laterally bend the trunk.