It's different for everyone, I think. When I sit for long periods, I'm more comfortable leaning back with my legs out or on a footstool. But when there's pressure on my parts, like when pedaling, forward leaning is the answer. It definitely has to do with the effort of pressing on the pedals as when I ride my bike with an electric motor (same bike just a motor installed) I can comfortably ride more upright. But then I'm spinning easily with very little pressure on my knees and hips. I may not get all the science/medicine behind it, but my body is VERY good at telling me when I get it wrong, lol.