The British Chiropractic Association has advised people to warm up before playing the Wii games. They are advising people to consider the Wii session as a form of excersise and hence do a warm up and a cool down. Players can then avoid “stiffness and possible injuries.”
The BCA has developed a ‘Wii Warm Up’ to help users retain nimble fingers but avoid over exertion and you can view it
here (pdf).