Used to be, a guy could sit comfortably on the couch and, by mashing a few buttons, make onscreen characters do all the hard work. Nintendo changed all that with the Wii. Suddenly, if you wanted to bowl or play tennis or help Mario save the galaxy, you had to stand up (gasp!) and move major muscle groups in a coordinated manner (heresy!). All those years of disciplined training to develop Thumbs of Steel (and Buns of Marshmallow), and Nintendo changes the game.

On May 21, the company launches a further assault on flabby fun-seekers with Wii Fit, a hardware/software exercise combo. The gadget part of this $90 package is the Balance Board, a sturdy platform, roughly twice the width of a bathroom scale, that communicates wirelessly with the Wii console. Inside are four strain gauges that precisely monitor the way your weight shifts while standing on the board. I was impressed with the device's sensitivity—it's built like a serious piece of exercise equipment, not a toy.

The software includes 40 exercises, broken into four categories: balance, aerobics, strength training and yoga. Tying them together is a comprehensive monitoring program that stores your results and graphs them over time. Unless you're in extraordinary condition, your first few minutes with Wii Fit will not leave you smiling. The software centers on body mass index (BMI), a ratio of weight to height that's supposed to indicate how much body fat you're carrying. If you've ever figured your BMI (with an online calculator like this one), you know it's an unforgiving, unrealistic bastard of a measurement. After the program tells you you're overweight or obese (trust me, it will), figure out how much you'd have to lose to hit "normal weight." In my case, even if I were to get down to a weight I know looks reasonable on me, I'd have to grow six inches, and exercises for spinal lengthening are strangely missing from the program. Still, the stat does offer a way to monitor your progress.

The Wii Fit exercises can be very strenuous, lots of fun, or a combination of the two. The balance category has an addictive game where, by shifting your weight, you bob your head to hit incoming soccer balls (and dodge cleats that one of your "friends" is apparently hurling at you). There are also two clever skiing games. A slalom course challenges you to aim yourself between the flags by shifting your weight. The ski jump has you crouching to build up speed and then standing up quickly to leap at just the right moment.

Aerobic activities are also ingeniously designed. I especially like the virtual hula-hoop challenges, in which you try to keep multiple hoops aloft by twisting those hips for all you're worth. Which brings up an interesting point: You are, periodically, going to look like an absolute moron while playing Wii Fit. I'm inclined to play in a darkened room with no mirrors.

I've never been a yoga kind of guy—they invented the Downward Klutz position with me in mind—but I am planning to try the Wii Fit yoga exercises, once I have that darkened room set up. And the strength-training exercises, which include arm and leg lifts, lunges, torso twists and such, will test the boundaries of even the physically fit, while letting the rest of us start off nice and slow.

The use of onscreen coaches (male or female, your choice) is a standout feature. The coaches do a fine job of explaining the required movements, demonstrating them clearly, and even suggesting what you’re doing wrong based on the board's feedback. If I were designing the program, of course, there would be a cheat code to unlock a nasty drill-sergeant trainer. But then again, I've been playing in-your-face videogames for a long time and expect to be abused by my software.

Most of us lack the time to go to a gym regularly and the self-motivational fortitude to work out at home. Nintendo has created a system that combines reasonable physical demands with powerful psychological incentives, and it deserves the sales tsunami that's about to hit when Wii Fit goes on sale May 19.