[RG_]

What you define as "holding position" can vary, also. For reasons already well explained, I also think it's unreasonable to expect perfect stillness. However, limits on movement (no standing up, keeping hands in front, etc.) can be imposed that allow room for reaction without being disruptive of the process. Not that even those are easy to always observe, but getting them to do so falls within the realm of reasonableness, firmness, and possibility to obey.