Muscle · Lumbar spine

Quadratus lumborum

Musculus quadratus lumborum

A flat, four-sided muscle filling the gap on each side between the last rib and the back of the pelvis — the only muscle spanning that window, which is why it is so often the source of a one-sided low back ache. It side-bends the trunk, hikes the hip up toward the ribs, anchors the twelfth rib so the diaphragm has something to pull against, and helps hold the lumbar spine steady. Clinically it is one of the two muscles the Momentum system lists as habitually tight in this region, and it is a named release target in the low back pain flow: when the glutes and deep core are not controlling the pelvis, QL takes the job and holds it all day.