Don't take this wrong, but my guess is that you're just weak in that area. Most folks are, especially the 50+. Those lower back muscles and tendons are all part of the posterior chain. When you push down hard on the pedals, the chain of force and therefore strain goes all the way up to your shoulder blades Google "posterior chain exercises" and start doing them, like all of them. Gym membership should be in your immediate future. It'll take a year to make substantial progress. Start light with high reps and slowly increase the weight or effort. Turning back the clock takes effort and concentration but it works.
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