The quadratus lumborum is a deep muscle of the posterior abdominal wall. It originates from the iliac crest and the iliolumbar ligament and inserts onto the twelfth rib and the transverse processes of the upper four lumbar vertebrae. It sits just lateral to the lumbar spine, nestled deep beneath the erector spinae and posterior abdominal muscles. Functionally, the quadratus lumborum assists in lateral flexion of the trunk when contracting unilaterally, and helps extend the lumbar spine when cont