The quadratus lumborum muscle is a large, quadrilateral-shaped muscle located in the posterior region of the abdomen. It originates at the iliac crest and posterior superior iliac spine of the pelvis, and inserts into the twelfth rib and transverse processes of the lumbar vertebrae. The quadratus lumborum muscle is one of several muscles that make up the posterior abdominal wall.

The main function of the quadratus lumborum muscle is to laterally flex (bend to the side) the spine and extend the trunk. This movement is essential for activities such as reaching to the side, or bending to the side to pick something up. The quadratus lumborum muscle also helps to stabilize the spine and pelvis during movement.

In everyday life, the quadratus lumborum muscle is used in many activities that involve lateral flexion of the spine, such as reaching for something on a high shelf or bending to the side to pick something up. It is also used in activities that require extension of the trunk, such as standing up from a seated position or lifting a heavy object.