Often referred to as the back muscle, Quadratus Lumborum is an enormous muscle that takes up a lot of space within the abdominal cavity. Because of its size, it rests in close proximity to many structures, such as the diaphragm, colon, kidneys, the ilioinguinal and iliohypogastric nerves, and each of the intrinsic back muscles. Quadrilateral in shape and narrower above than below, the Quadratus Lumborum is the deepest abdominal muscle.
The Quadratus Lumborum supports movement of and adds stabilization to both the pelvis and the spine. Moreover, it pulls the torso downwards (lateral flexion) when activated on only one plane. Also, it is an accessory muscle to the breathing process, fixing the 12th rib during forced expiration. Innervation occurs via branches of the T12 to L4 ventral rami.