Ya mean Gym Member?
( Sorry if I spoiled it :/ )
It just Maxes your hp to 5 more points (I think)
Obviously I know what gym member is, I mean the actual gym map you can access when you beat the game a few times.
did you read my hint, or are you like me who only knew about the one punch anime due to references :P
Dude, I don't watch anime.
oh...
well the names of some upgrades tell you the exercises