The Wii Balance Board measures shifting weight.

For the past eight days, I’ve been stretching, crunching, yoga-posing and even running using this $90 package from Nintendo (NTDOY.PK). Wii Fit is the title of the disc that comes with the Wii Balance Board, a sturdy platform on which you perform your routines. The two are used together for various types of yoga, strength training, aerobics and balance games, which involve the Wii game system’s “Miis” — on-screen representations of yourself — that interact with trainers and other virtual characters. Sensors in the Balance Board detect a user’s weight, body mass index, balancing skills and positioning during activities, and the Wii Fit program keeps track of this information, providing tips on technique or weight loss. Wii Fit and the Balance Board must be used with the original Wii system, which costs $250.

I’ve grown fond of using Wii Fit and the Balance Board because it holds me accountable for my weight and balance skills. I learned fitness and health tips from a small, animated image of the Balance Board that jumped around on-screen — such as the fact that people who cross their legs while sitting are more likely to have back problems. And after a few days of using the system, I could feel a difference in my muscles.

For those used to 30-to-40-minute workout sessions, the Wii Fit programs may seem lightweight, since activities last for only about three to five minutes each. To unlock longer activities or additional strength-training repetitions, you must first do them a handful of times in their short versions, which can be frustrating. Nintendo says this is designed so that average users don’t feel intimidated, but I felt like some of the activities ended just as I was getting into them. Almost all activities involve actually standing on or touching the board.

Motivation is a key element in the Wii Fit programs. An on-screen graph tracked my progress, and I “stamped” each calendar day to show that I had exercised. Each minute of activity added a point to my Fit Bank, and enough points unlocked new activities; Wii Fit includes over 40 altogether. On-screen instructors demonstrated and joined me during yoga and strength-training exercises, congratulating me when I held my balance or noting that I stopped mid-exercise.

A daily body test measured my weight and body mass index against those from previous days and challenged me to two short balance tests, which changed daily. After, I was told my Wii Fit Age, a number reached using my actual age, BMI and performance on the balance tests. On good days this number was a couple of years below my real age, but it drifted upward when I was given harder tests, sometimes reaching 13 years older than my actual age.

Top: Wii Fit charts fitness progress over time, including body mass index and weight. Yoga (middle) and balance games like Ski Slalom (bottom) mirror board movements.

More than one person can use the system and profiles can be password protected. Friends can use the Balance Board under someone else’s profile; the system notices weight changes and confirms that it’s alright to continue.

The Balance Board measures roughly the size of a step used in step aerobics classes, and uses the same technology that gauges the weight of airplanes. These sensors expand and contract when someone stands on the board, measuring weight, and where and how that weight is shifting on the board. This works out of the box with four included AA batteries. Its power button can be nudged on with a foot, turning off automatically after five minutes of non-use.

I started with basic yoga, attempting the half-moon pose. I opted for a male instructor who told me that this position helped improve posture and digestion. He briefly demonstrated the position, and we got started on the actual session, which lasted two minutes. A large, on-screen circle that expanded and contracted illustrated when I should inhale and exhale as I held positions. I tried the Warrior, Tree and Sun Salutation positions throughout the week.

In the strength-training section of the program, I liked the Single-Leg Stretch exercise, which involved standing on the Balance Board on one leg and holding the other leg off the ground, bent up toward my chest. I slowly extended this bent leg out and in, while moving my arms and trying to maintain my center of balance. This exercise started at six repetitions, but increased to 10 reps as I improved; 20 reps are the maximum.

I tried the Torso Twists and Rowing Squats, but had a lot of trouble with Push Ups and Side Planks (modified push-ups), because I’m not good at push-ups. To my surprise, my trainer didn’t notice when I couldn’t finish the session. I got my best score on that exercise, clearly a flaw in the system.

During yoga and strength-training exercises, an on-screen red dot marked where my center of balance was detected, and I was encouraged to try to keep that dot within a highlighted area. After the exercise, a diagram showed where my weight had shifted, and I earned better rankings when I distributed weight evenly.

Aerobics were more fun, including hula hooping. I rotated my hips and I leaned forward so that my on-screen cartoon self could catch hoops thrown over my head by other characters. The Balance Board tracked the number of times I rotated my waist around in a circle. One aerobics activity that didn’t use the board was Basic Run, which requires users to put the Wii remote in a pocket or hold it while running in place for time intervals measuring three, five or 10 minutes depending on the pace. This mode is designed so users can either watch other runners on-screen or tune into television while running as Wii Fit tracks your progress. An aerobics activity called Basic Step was like Dance Dance Revolution: on-screen footprints showed where and how to step next — on and off the board.

Balance Games were fun — but hard. I tried a bunch, including Soccer Heading (where you pretend you’re the goalie), Ski Slalom and Tightrope Walk, and was surprised by the sensitivity of the Balance Board. But the more I played, the better I became at controlling my balance.

I brought the Wii and Balance Board with me to my parents’ house last weekend, but most people will do best to keep this board in one place since it weighs 10 pounds.

When I stepped onto the Balance Board at my parents’ house in a room with wall-to-wall carpeting, the system thought I had lost 13.4 pounds since the day before. I stepped off and tried this measurement again a few minutes later, but was told I lost another 4.6 pounds. However, when I returned to my own home with the board on an area rug on hardwood flooring, the system showed I “gained” 16.1 pounds. My weight at my own house was on the mark.

Some of my relatives jokingly saw this instant weight loss as a reason to buy a Balance Board, but this made me question the system’s accuracy.

Nintendo solved the mystery: The carpeting in my parents’ house, which I mistakenly thought was similar to the area rug in my house, must have been touching the bottom of the board, therefore transferring my weight onto the carpet and away from the board. Four circular “feet” come with the Balance Board to elevate it, solving the problem.

When the Wii Fit system thought I lost or gained a lot of weight in one day, it was concerned and offered tips for healthier fitness. In the case of the supposed weight gain, I was asked to think about why I might have gained weight, then had to select an answer from a list of reasons why, including Late Dinners, Night Snacking and Not Exercising. The system gave health tips related to the reasons.

Users’ outside exercise can be accounted for in the system. I added the time I spend playing tennis each week and this information plugged into a graph to illustrate fitness activity over time.

The idea of having a virtual trainer and a way to set weight-loss goals while tracking progress could be truly valuable for people in need of motivation without the expense and/or hassle of going to a gym. If you’re looking for a fun way to get in shape, the Balance Board will do the trick. Like the Wii, its activities encourage all sorts of people to use it, marking yet another smart move from Nintendo.

• Email mossbergsolution@wsj.com