Well, the woman in your exercise class was in a very distinct position. Technically, you had authority over her. Also, you had able-bodied privilege compared to her in the respect that you didn't suffer from arthritis. So it was entirely valid for her to speak up and offer criticism since she was the recipient of a public service, especially since it was coming from a place of pain (both physical and emotional).

That's way different from someone who begins viciously berating someone else in an unsolicited manner. In that case, it's entirely reasonable for the target to stand up for themselves without being gaslit as "defensive" or "fragile."