Dr. Julio Bonis couldn't figure out why his right shoulder was so sore after awakening one Sunday morning. He hadn't suffered any trauma or recent injuries, which made the pain even more puzzling.

After consulting with a colleague, the 29-year-old family practice physician in Madrid, Spain, determined that he suffered from acute tendonitis caused by what Bonis calls "Wiiitis." Yes, the popular Nintendo Wii video game console has a dark side.

In an e-mail interview, Bonis said Friday the injury stemmed from playing tennis on his new Wii the day before. Using the console's motion-sensitive controller, Bonis spent 10 hours returning serves and volleying with an imaginary opponent. "I was surprised at the intensity of the injury," he said. "Because I was playing a video game, I did not take the usual preventative measures before exercising -- warming up and so on."

In a letter published in the New England Journal of Medicine, Bonis coined the term "Wiiitis" to describe injuries relate to the popular console, which is outselling by a wide margin Sony's PlayStation 3 and Microsoft's Xbox 360.

Injuries caused by Wii play are slightly different than those connected with real sports, Bonis said. "With the Wii, the user doesn't feel resistance to the movement. The biomechanics of Wii movements are different than those of tennis or other sports."

In his case, Bonis managed to stress the tendons in his shoulder to the point where he wasn't able to lift anything without causing pain. The discomfort lasted three days, and he wasn't fully recovered until five days of Ibuprofen and shoulder rest.

Bonis has some advice for people planning on grabbing the Wii control for a bit of tennis, air guitar, or any other physical activity: "I recommend warming up the shoulders, or any other body part involved in the game," he said. "Play with moderation and respect the limits suggested by the Wii software."

Bonis, however, acknowledges that his advice may be tough to follow, given how addictive the games can be. "I'm trying to follow my own recommendations, but it isn't easy."