In Rochester, Minn., Jeremy Scherer and his wife spent three hours playing tennis and bowling, two of the games included with the Wii. Mr. Scherer says he managed to improve his scores -- at the cost of shoulders and back that were still aching the next day. "I was using muscles I hadn't used in a while," says Mr. Scherer, a computer programmer who describes himself as "not very active." Mr. Scherer is vowing nightly "Wii workouts" to get in better shape.

Another hazard: collisions. All those flailing arms can sometimes inadvertently smack into lamps, furniture and even competing players. IGN.com, a popular site that reviews videogames, said one player testing the Wii lost her grip and sent the controller flying into a wall. Blaine Stuart of Rochester, N.Y., mistakenly whacked his fiancée, Shelly Haefele, while playing tennis and also accidentally hit his dog while bowling.

Nintendo itself warns players about this risk just before some of the games begin. A message flashes up on the screen saying: "Make sure there are no people or objects around you that you might bump into while playing." Some Wii games also have pop-up reminders every 15 minutes advising gamers to take a break.