What is quadratus lumborum?

Some articles on quadratus lumborum:

Lumbar Plexus - Branches
... on its proximal lateral border to run laterally and obliquely on the anterior side of quadratus lumborum ... closely follows the iliohypogastric nerve on the quadratus lumborum, but then passes below it to run at the level of the iliac crest ... branches T12-L4 • Psoas major • Quadratus lumborum • Iliacus • Lumbar intertransverse Similar to lumbar plexus there is brachial plexus in upper limb ...