Nintendo never claimed it would, of course it is nothing like playing the real sports or doing proper excercise, In Wii tennis you don't run around and the remote is not as much effort as a racquet [sp?]... Only idiotic press claimed the Wii was a substitute for the gym or proper excercise.
If though you merely replace your old videogaming time (fingers and thunbs) with the Wii sports way you will of course be using more energy and losing more weight/getting fitter... this is obvious because you move more (assuming you don't play Wii tennis like i do:- sat on my bed).
I am glad you posted this kber because I have been seeing the various articles from Google News about it, and wanted to comment on how stupid journalists are but I usually have to sign up to comment on those sites.
