Are you having stress strain on your lower back? I have been thinking about you issue when riding as I have had a flare up with my osteoarthritis in my back and that has me off of 2 wheels onto my recumbent trike now and often several times a year. I have no issues with injuries or structural abnormalities but arthritis can bring structural abnormalities to the back, It seems to me something is amiss with the set up as I am hyper sensitive to back issues and have zero issues in my recumbent. My general position is as any shop I believe would recommend, seat back a comfortable angle that provides good lumbar support, full contact with the backrest for solid support and hips level while pedaling w/o rocking. I know some folks like a what to me is a very aero position but I like being more upright enjoying the view w/o strain.