Embarrassing question, I guess.

This is a bit embarrassing, I suppose, but I originally became interested in tennis last year because of the movie Challengers, due to it being scored by Trent Reznor and Atticus Ross ~(and because I'm bi af)~, so apologies if this point is inappropriate (a couple of screenshots of a shirtless guy, I'm tagging it NSFW just in case) or if this is a dumb question, but what are these blue tapes (?) on the character Art's back? I looked everywhere and couldn't find it and I've never noticed a real tennis player wearing something similar. Thanks. 🙈