LOL. Glad this happens to other people.
I use my son's when he brings it over, so that's my excuse for getting sore - I don't get to play often enough.
The first and second times I played, my arm was really sore but so was my
butt. 
I played so hard at tennis and bowling and must have stood in a way that put stress on that particular part.
It really is fun - I even sweat playing tennis cause I move around so much. A video game that gets you moving? Can't be all bad.