The quadratus lumborum or QL is a common cause of back pain which is to one side and comes on after lifting or twisting.
Origin : Posterior iliac crest.
Insertion : Twelfth rib.
Transerve processes of L1-L4.
Actions : Lateral flexion (side-bends) of the trunk.
Innervation : Ventral rami of the subcostal nerve.
Daily uses : Bending sideways to pick something up.
Example strengthening exercises : Side bends.