If we interpret "All my bones shall say…" to mean that a person sway during recitation, it was not meant to refer to prayer but to psalms and other parts of the service, and not to the ‘Amida [Eighteen Benedictions]. A person who sways cannot focus their intention, whereas a person who stands steady can increase their focus, this is verifiable. Also, it is not appropriate behavior to sway before the King as might a tree in the forest before the wind, and it says "…acacia trees, standing" – Seraphim stand as do rooted trees.