The Quadratus Lumborum (QL) is a deep core muscle located on both sides of the spine that connects the iliac crest to the 12th rib and lumbar spine. Its primary functions are to unilaterally flex the spine laterally and working together to extend the vertebral column. It also helps to anteriorly-tilt the pelvis.
This muscle is important (aren’t they all?) not so much for where it is or what it does, but rather what it ends up doing and the problems that occur when that happens. The QL is primarily an “assisting” muscle, meaning that of the listed functions, there are other muscles that perform the movement better.
For example, the erector spinae are the primary lumbar extensors in our bodies. When the erectors are compromised (i.e. weakened) because of a traumatic incident or excessive sitting, the QL has to help out. To provide stability, the QL are pretty much contracted for a long period of time (hypertonicity). Hypertonicity, over time, will cause muscles to spasm, eventually weaken and possibly form adhesions. All of this leads to low back pain! In this instance, the low back pain is caused by the QL, but the original culprit was a weakened erector group.
The same thing can happen when the hip abductors (gluteus medius and gluteus minimus) are weak. The abductors move the leg away from the midline of the body and stabilize the hips when walking. When the QL takes over for weakened abductors, the results are the “Frankenstein walk” and low back pain.
Keeping the QL strong (best exercise: side planks) is imperative. But what this really shows is that (1) you need to strength train on a regular basis and (2) your strength training program must address every muscle group to prevent imbalances.