"They'll probably take issue with Wii Fit simplifying the whole discipline too much for their likes," he explains.

If you select strength training, you can choose between single-leg extensions, torso twists, lunges, jackknifes, and a push-up/side plank combo. The balance board can't tell you whether you're doing these exercises correctly. It can only count how many times you do them (when you touch the board) and offer general suggestions, such as an admonition not to drop your hips during the side plank.

With aerobics, you can start with a basic step, a basic run (in place, off the board, with the wiimote in your packet) or hula-hooping. These all require a low level of cardio activity, and Bryant says he was the least impressed with this part of the game. But the farther you advance in the game, the harder these exercises become.

"With the aerobics and even the strength training, the activities become more challenging and they encourage you to do more repetitions, so there is a built-in progression," Bryant says.

Throughout Wii Fit, a trainer walks you through demos, then the actual exercise. She or he (your choice) offers encouraging phrases, like "Good job!" or "You're strong." The onscreen trainer also makes comments, like a suggestion to shift your weight or, after a poor attempt, to keep training in order to improve.

Each time you weigh in, you receive a "stamp" for the day. A "fit bank" logs how many minutes you participate each day, as well as your ongoing weight and weight goals. But it does not log calorie burn, which might lead people to think they are exercising harder than they really are, says Bryant.

"It's a nice idea to take advantage of technology that's typically associated with sedentary behavior and use it with some of the gaming aspects, particularly for young people," he says of video fitness gaming. "The one caveat is that while that's certainly better than the alternative -- traditional video games -- one shouldn't use it as a substitute for the real thing."

Fitness Games: Entertainment With Activity

Nintendo isn't claiming that Wii Fit will help people lose weight -- or even become healthier. The company says it merely hoped to create a game that combines entertainment and the ability to track progress with a healthy activity.

"We hope that Wii will encourage users to be more physically active as well as spark a discussion about fitness in the household," Franklin says. "As with any other exercise, the effect has many variables, depending on the person who is working out and the level of activity."

Bryant says Wii Fit isn't likely to replace regular trips to the gym -- especially for those who are already fit.

"It's great for the person who hasn't been doing a whole lot," he says. "But for individuals who exercise quite a bit, it isn't going to be a huge challenge."

However, he adds, "I do think it has the ability, if you hang in there long enough, to be a bit more challenging for those who are already engaging in physical activity. It's not a substitute, but it could certainly be a nice complement to a regular exercise program."

ACE plans to release a study in June on the calorie expenditures of each of the original Wii sports games. The study will show, for example, that playing Wii golf burns about 3 to 3 1/2 calories per minute, Bryant tells WebMD. Walking a real course and hitting an actual ball, by comparison, burns twice that many.

Likewise, a friendly game of tennis burns about 8 calories a minute, while Wii tennis burns about 5. That's not bad for a game, Bryant says, but it still represents almost a 50% difference. The study concluded that boxing is by far the most strenuous of the original Wii games, burning as many as 7 calories per minute, compared to 10 to 10 1/2 for actual sparring.

"I've watched people play the Wii boxing, and they're just wiped out," says Bryant. "There's a lot of local muscle fatigue that occurs, but the caloric expenditure doesn't come close to how intensely you feel you're working. That's 210 calories for 30 minutes -- that's not a whole heck of a lot. The load isn't enough to count as strength training, but it is enough to count as muscular endurance."

Can Exercise Video Games Ever Replace the Gym?

Joseph Donnelly, EdD, an exercise physiologist and a professor of health sport and exercise science at the University of Kansas Center for Physical Activity and Weight Management, is skeptical about lab studies of calorie burn during video game exercises.

"You can take someone and stick them in a lab and get them to play games and expend a certain amount of energy, thus creating the circumstances whereby the energy expenditure appears to be promising," he says. "Yet when these things are translated to the general population, it's not so good."

Donnelly also points to the downward spiral of physical activity in this country, and says that all of the exercise recommendations are currently being ratcheted up -- not down. The CDC, Institute of Medicine, and American College of Sports Medicine (ACSM), for example, are re-examining their guidelines to determine whether they should increase the recommended amounts of physical activity.

The ACSM currently recommends at least 30 minutes of physical activity per day. But Donnelly, who is involved in a study of these guidelines, says that not only are they too modest, but they have also been "grossly misinterpreted" by the media and the public.

"We need people to be moderately active throughout most of the day," he says. "Prehistoric man engaged in moderate activity for six to eight hours a day. Those are the genes that we inherited."

Like Bryant, Donnelly cites Konami's Dance, Dance Revolution, an active game that made headlines when it was released, then quickly began gathering dust in many households. But even if people do stick with exercise games like Wii Fit, he says, the time typically spent using them is rarely enough to make a difference.

"Electronic gimmicks do not appear to be the solution for the physical inactivity problem that we have in this country," he adds. "You're not going to be able to play a Wii game for 15 or 20 or even 30 minutes and get the kind of energy expenditure that is the same as six to eight hours" throughout the day.