Now for the serious post.
I do hope there is a fitness requirement... but perhaps not the type that the OP had in mind.
Once, after watching multiple (4 or more) wheelchair-bound PAX boarded onto a plane, I asked the FA about what happens to these folks in an emergency deplaning.
She explained something about using blankets as stretchers to get the PAX to the doors.
So, I hope there is something along the lines of some minimum strength requirement for FAs. I've seen some very petite FAs.
Can anyone "on the inside" provide some perspective here?