Some auto-immune and connective tissue inflammation can be related to reduced collagen in the body. The appearance of my hands and skin doesn't change with blood circulation or gravity. I get plenty of natural collagen from a diet rich in animal protein and low in carbs/sugars, so there's not much more can be done about it if it is collagen related. The change in skin thinning was fairly sudden and only started after the PMR and prednisolone did, so I think it is less related to ageing, though there may be a little of that.

I'd warn against the idea that collagen supplements will help. In my case, I'm almost certain that hydrolysed marine collagen powder prompted my initial PMR onset with the PMR suddenly starting 5 days after I started taking it. Hydrolysed collagen is not collagen in its natural form. It is altered collagen which contains a lot of hydroxyproline which converts to oxalates during digestion. Oxalates combine with calcium in the body and can cause problems with kidneys, and calcium oxalate crystals can also deposit in and around joints causing pain. So I stay away from processed collagen supplements and also high oxalate foods.