In addition to an old injury, my greatest problem was overly tight and weak glutes, which were pulling on the lower back structures to the degree that I could not climb a hill for longer than a quarter mile without having to stop, get off the bike, stretch, get back on, stop, get off the bike, stretch, get back on, rinse and repeat for a long as the hill was. Stretching and light exercises have helped tremendously, and the only time I have back pain now is on an exceptionally steep and long hill, which even around here I don't do often.