[TA] flat muscle of posterior abdominal wall; origin, iliac crest, iliolumbar ligament, and transverse processes of lower lumbar vertebrae; insertion, twelfth rib and transverse processes of upper lumbar vertebrae; action, abducts trunk; nerve supply, ventral primary rami of upper lumbar spinal nerves. SYN: musculus quadratus lumborum [TA], lumbar quadrate muscle, quadrate muscle of loins.