Disclaimer: I am ignorant in medical (and many other) matters. I have no idea what causes anyone else's back pain. I can only give my understanding of what makes MY back ache:
My healthy spine has a hollow shape in the lower back. This slight inward curve is called "lordosis".
http://orthopedics.seattlechildrens.org ... rdosis.asp (Too much is also not good, aka hyper-lordosis.) If the correct degree of lordosis is not maintained, and the spine curves forward at the top for long periods of time, the gelatinous discs between the vertebrae are squeezed out backwards until they rub on the nerves that travel along the spine and behind it. This causes back pain. The answer for me is to maintain the lordosis. While seated on an office type of chair, try hollowing the back and pushing the stomach forward, and then reverse the action by pulling the stomach back in and putting that convex bend in the spine, the one that causes the pain. Do this slowly a few times, and note that the pelvis tilts back and forth with this motion. To prevent the pain, the lordosis must be preserved by tilting the pelvis forward while hollowing the back. This can be aided by lifting the rear of the seat, and ensuring that the front of the seat is not too high. After that, my main way to maintain lordosis is constantly to push the navel forward while paddling, imagining that the navel ("the core") is the driving point, rather than the shoulders or the feet. My back then hollows. If I forget to do this, my back aches horribly. If I maintain lordosis, I don't have the problem. I don't need a backrest, nor do I feel that a backrest is a long-term fix, for ME.
Nick