Wii shoulder pain

I have discovered that while it is unbelievably fun to play tennis and stuff with the Wii, it is also exercising muscles I have not exercised in probably 20 years. The game causes you to replicate the movements that are performed when the sport is played. That means that my shoulder muscles hurt after playing tennis for half an hour a couple of days in a row. I still love it, don't get me wrong there, but there is a definite need for some moderation on use to avoid having the shoulder hurt until it is used to the new muscles being used.