Yes, open to interpretation, within reason, however, a hobby isn't really the same thing and again, that is information for the PS, not the EC section, but feel free to list whatever you want, I would not, however. Again, without some kind of external accountability, you could put pretty much anything on your app as an EC then...doesn't mean its true or real and it looks like you are padding your app in my opinion, but feel free to add whatever you want and take the chance.
You can look at any number of pre-dental pages for various colleges and you won't find mention of exercise as an EC.
