It's not a matter of how strong your muscles are, becuase after the pressure in your bladder builds up past a certain point an automatic part of your body will simply override you trying to hold it in and simply release your bladder.
The woman in the hold your wee for a wii died from "Water intoxication". It's when you drink a MASSIVE quantity in such a short time that you dilute your blood.
http://chemistry.about.com/cs/5/f/blwaterintox.htm
And yes holding frequently for too long is a problem, so it is not recommended you do this dare every single day for a whole week.