Quadratus lumborum is attached to the iliolumbar ligament inferiorly by aponeurotic fibers, and to the adjacent portion of the iliac crest.  From these attachments the muscle extends to the medial half of the lower border of the twelfth rib.  Medially, it attaches via four small tendons to the apices, anterior surfaces and vertical ridges of the first to fifth lumbar transverse processes.

The muscle is enclosed between the anterior and middle layers of the thoracolumbar fascia.