For what it's worth I'm a certified personal trainer and have a degree in exercise science. In terms of cycling I've only been doing so competitively for 4 or 5 years.

I broke my back at age 16 when a rope swing broke and I landed on a vertical metal pole. My PT and RT has always since then been to train a strong back with muscle to give me extra support. When my back aches (when I'm out of season and being a couch potato) I return to the gym and work on my entire core, not just my back. Shoulders, upper back, lower back, abdominals, obliques, glutes, and hip flexors.

When obtaining a bike fit inform your fit specialist that you have prior back conditions. They generally find a less stressful position for your back. We're not all slinky professionals training for 4 hours a day in that god awful position. Before your rides, try stretching to loosen up those muscles. If you're out of the saddle often, support your torso with your hands over the bars, tighten your core, and keep your back straight. With enough practice you'll engage those muscles naturally and your form will be as smooth as butter. If you're riding for long periods of time, try pressing your shoulder blades together, this activates your back muscles naturally and promotes better posture. May not work for everyone but it does for me. When in doubt, ride in the tops and relax. Enjoy your ride, no need to suffer the wrong way.