i'm 5"11 with a 34" toe-hang blade putter (fitted with an arcing stroke). i have very wide shoulders. my natural, "loose", posture rounds my shoulders inward (picture 1) and is most comfortable. i've also tried putting with "good posture" where i keep my shoulders back (like i'm position to do a lat pulldown) and my arms are basically stuck to my rib cage (picture 2). this way is a little uncomfortable due to my wide upper body though i do think it provides the most stability when rocking the shoulders. i've been going back and forth between the two, tho i'm not sure which one i should commit to. any advice?
also, what muscle starts the backswing? is it the trail shoulder pulling up or the lead shoulder pushing down or a combination of both?