Sweat popped out on the reddened faces of 12-year-old Jakob Njos and 15-year-old Emily Njos, evidence that they had raised their heart rate while holding remote controls and punching the air as they stood in front of a television set.

The siblings had just finished a round of virtual boxing, one of several Nintendo Wii Sports games they got for Christmas. Onscreen, customized Mii cartoon figures representing each of them "fought" until there was a knockout. Sweat flew from the faces of the virtual characters, too.

"They don't take any rests when they play these sports so it's an endurance thing," said their mother, Siri Njos.

She is a physical therapist and knows that making video games physically engaging has its pluses. But she cautions it may have its minuses, too, if players don't use proper form and common sense.

From friends and family, she has gotten reports about pain and soreness among both adults and children who play Wii Sports.

"Adults get hit the hardest, but even kids are complaining of pain," she said.

Wii Sports includes bowling, baseball, boxing, tennis and golf.

Imagine punching the air as if boxing or sweeping the air as if hitting a tennis ball hundreds and hundreds of times.

"You may hit a thousand tennis balls in a given period of time, something you'd never do on the tennis court," said Dr. Mark Klion, a sports physician and orthopedic surgeon at Mount Sinai School of Medicine in New York City. "This is repetitive activity you clearly wouldn't do in a regular sports activity."

Last year, when Wii Sports came on the market, a doctor in Spain wrote a letter published in the New England Journal of Medicine describing shoulder tendonitis caused by playing hours of tennis on his Nintendo Wii console. He dubbed the condition "Wii-itis."

People can play virtual tennis for inordinate periods of time because "unlike the real sport, physical strength and endurance are not limiting factors," Dr. Julio Bonis wrote in his letter.

Klion has encountered patients with shoulder, elbow and wrist strain from Wii overuse.

He said what they injure is soft tissues - muscles, ligaments and tendons. There is microscopic tearing of the tissues, which causes pain and inflammation.

Symptoms may not appear for 24 to 48 hours, Njos said. The shoulder is particularly vulnerable because it is a big, mobile joint but the least stable of our joints, she said.

"I have nothing against these games, I'm just wanting to send out a buyer-beware message," she said.

Njos preaches good form, not swinging or punching full-out and not playing the games for hours at a time.

Research published last month in the British Medical Journal said children who play video sports on the Nintendo Wii burn more calories than they would playing regular video games. But the research also said children who played actual sports burned more calories than Wii Sports players.

Klion said his family couldn't resist getting the popular Wii gaming console, which was one of the hottest gifts this holiday season.

"I'm an active user myself with my kids," he said. "It's not as good as running outside and playing imaginary games, but it's much more active than PlayStation and Xbox and sitting on your rear end doing nothing."

More physically engaging, interactive games are on the way from Nintendo. The USA debut of Wii Fit games is expected during the first quarter of the year. Wii Fit includes a Wii balance board attachment and fitness games such as the balance game, yoga and muscle training.

Gannett News Service, Sam Upshaw Jr./The (Louisville, Ky.) Courier-Journal

Emily Njos, 15, and her brother, Jakob, 12, box one another during a game on the Nintendo Wii in Louisville, Ky., on Jan. 2. Emily uses an unorthodox technique, which can lead to injuries over time while her brother uses a good form, keeping his elbows in closer to his body.