I am not fit... yet. I have lost 65 lbs over the past 18 months and have another 40 to lose. My core could use a lot of work.
When I bought the bike it was fitted to me.
I would say that when my lower back bothers me I go upright and that is when I lock my elbows. That just occurred to me when I thought about why I stay up. I do like to go down into my drops but not when my back is sore. So a bit of a revelation. My back didn't bother me at the start of the summer and my elbows did not either.
As I lose over the winter I am hoping that this will help my lower back. I suppose I could do the exercises that I saw my athletic father do every night for his back.