Stuart McGill is doing some amazing research out in Canada about the mechanics of the low back, low back pain, improving low back function and back performance. His latest study is pretty cool looking at the strongest guys on the planet. Professional strongmen. He was looking at analysing the strong man yoke carry. He also test the hip abductor strength on each athlete. What he found was that none of the athletes hips were strong enough in theory to stabilize the pelvis and move the leg under the loads they were lifting in competition. So where was the strength and stability coming from?
McGill tested more and came out that the quadratus lumborum was indeed helping and bridging the gap in the performance. The QL was indeed a big performance muscle.
To train the QL you have to be doing some unilateral carry type work. Single arm farmers walk, single arm waiters walk, suitcase deadlifts ect. If you're not doing any single arm weighted work, add it in and watch performance and in theory back pain improve.