Originally Posted by Carbonfiberboy

You are very well educated and I've been riding bikes for 70 years and am not totally uneducated. Right now for obvious reasons I'm researching the age-related loss of skeletal muscle mass and function.

One does not want to ride with a curved back. Rather roll your pelvis forward so that instead of sitting on your ischial tuberosities, you are perching on your inferior pubic ramus. Your back should be almost straight. Modern saddles are designed for this posture. The point of this is not aerodynamics as many people assume, rather it's comfort. When the bike hits a bump, the spine is thus loaded in bending (flexion) rather than in compression. This is actually helpful for the back muscles and issues that we all get eventually, like thin discs, fractured vertebrae, arthritic facets, and lumber stenosis - I have them all. I don't have a ruptured disc but I ride with people who do. The idea is to reduce compression loads. Of course the back muscles will load the spine in compression in trying to prevent this bending. This is undoubtedly true but it's a more gentle pressure than the compression of a vertical spine when hitting a bump. I think the muscular response is useful in that one's back gets stronger and thus less affected by the strains of daily living. I can't count the people who've told me they've injured their backs by making a slightly wrong move with their bodies. That shouldn't happen.

Ski joring with dogs looks like a lot of fun. I've seen it but not done it. Raced Nordic in my youth. Also wonderful back exercise!