Identify an easily missed muscle at lower back pain !

Quadratus Lumborum(QL) is a muscle connecting lowest rib with top of pelvis. As shown in the image, it is like a bridge between middle back and hip so is involved in most of lower back movement such as bending and twisting.

The muscles is easily missed because it lies so deep. A better position to identify this muscle is patient lying on unaffected side, with arm of the affected side up behind the head. If you feel so painful when being pressed at the spots indicated as X , especially the upper one,  it is more likely the muscle got involved in your lower back pain.