The psoas minor is a weak flexor of the lumbar vertebral column and is innervated by the anterior ramus of nerve L1.
Laterally, the quadratus lumborum muscles fill the space between rib XII and the iliac crest on both sides of the vertebral column (Fig. 4.142). They are overlapped medially by the psoas major muscles; along their lateral borders are the transversus abdominis muscles.