I have chronic back pain and stiffness due to a fairly severe scoliosis (spinal curvature). There are times that I get pain while riding - in the vicinity of the quadratus lumborum muscle, and triggered by fatigue from holding myself upright in a good riding posture for extended periods of time - but that only happens now and then. More often, riding makes my back feel a LOT better. The movements involved, both in the actual riding and in the horse grooming before and after, do a lot to loosen up and mobilise my often-stiff muscles. When I've been riding a fair bit, I find that my back's generally a lot less painful and more mobile than when I've been less active.