Hmm... I don;t know.

There is many cases where there is poor posture, or for example segmental stiffness that does not necessarily result in pain.

Pain is the result of structural overload, compression, tearing, shearing, etc. which do not automatically manifest due to poor posture. While the physical limitations of poor posture are various, it is not pain that is the only limiting factor, but the one looked most closely into due to the associated misery.

However, having said that, the reverse of opposite (that return to normal upright posture against gravity) with a subsequent increase in function or decrease in pain appears to be more fundamentally true.