You might want to check some of the US government websites (.gov)for that. I’d start with the Dept. of Labor & Census Bureau. The term “fitness instructor” is pretty broad. To tell you the truth, I haven’t heard that term in a long time – I know that’s what we were called in the 80’s & early 90’s. Then the terms “personal trainer” & “fitness professional” came into vogue. I actually classify myself as a “fitness consultant” on my tax returns. Just curious – why do you want to know, is it for a project or paper?