Proximal Attachments
Erector spinae has a U-shaped origin.  It arises from the anterior surface of a broad thick tendon inferiorly, which attaches to the median sacral crest, the spinous processes of L5 to T11 and their supraspinous ligaments, the medial aspect of the dorsal part of the iliac crest and the lateral sacral crest.  Deep to the lateral limb there is a fleshy attachment to the iliac tuberosity and the inner lip of the iliac crest.  Muscle fibers pass superiorly and divide in the upper lumbar region around the level of the twelfth rib into three columns, the iliocostalis laterally, the longissimus intermediately and the spinalis medially; each of which is further subdivided.  

Distal Attachments
Iliocostalis lumborum passes superiorly and laterally from its origin, to attach by a series of flattened tendons to the inferior borders of the angles of the seventh to twelfth ribs.