Hah! How about this: So they don't end up injuring themselves.
There's a reason why ergonomic chairs exist and why they all follow the same exact design rules (to varying degrees).
Most important features, in order of importance:
- Adjustable arm wrests. Height is more important than swinging out/in. The out/in feature is more for convenience of getting them out of the way on occasion than anything to do with ergonomics.
- Adjustable height. The base of which all ergonomic chairs were developed.
- Adjustable lean. Meaning: You can control how far back it tilts.
Also important: Men and women have different ergonomic requirements! A woman's spine is curved and should not have the chair configured the same as a man.