Originally Posted by gugie

Those Turbotrainers lock down the rear wheel at the dropout, so side forces flex the bottom bracket more than normal, and you notice it more since you don't have to pay attention to where you're riding.

531, SL, CrMo, any steel has the same modulues of elasticity (they all flext the same), differences in stiffness are due to geometry (wall thickness, tubing diameter, butted section length).

And, as Salamandrine notes, frames don't wear out and get less stiff, unless it develops a crack somewhere, which is somewhat rare.