There was a wide variety of attires on display on my test(s): shorts, thermals, under-skin armour, long and short-sleeved shirts. As the above says, I think as long as the spirit of "sportswear" is met, the specifics don't really matter.
I know on my IOT Famvisit test, some chose to use strappings on their ankles but they were obscured by socks so, if you need a knee strap, perhaps consider tights underneath the shorts or whatever you feel comfortable with.
Can't comment on openly-displayed braces as I haven't witnessed any personally.