When it comes to New Year's resolutions, year after year the No. 1 spot is most likely occupied by "lose weight. " Its persistence probably...

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When it comes to New Year’s resolutions, year after year the No. 1 spot is most likely occupied by “lose weight.” Its persistence probably has something to do with the fact that most people have trouble actually succeeding.

There is a simple reason. People understand they have to burn more calories than they take in, said John Jakicic, a professor at the University of Pittsburgh and a researcher in the field of exercise and long-term weight control. But “most people don’t know how many calories they burn a day,” he said. “They have no clue.”

Getting a clue — or at least an accurate estimate — used to require a visit to a laboratory or the use of complex scientific equipment. Now there are simplified electronic monitors designed to accurately gauge physical activity and the calories burned, which is the silver bullet for weight loss. “There is a lot of evidence that shows they work,” Jakicic said.

How well a monitor works depends on how much it is used, which boils down to personal taste. We tried four of the more common products.

Striiv ($100): Just released in October, the Striiv is designed with the sedentary in mind. It is an ultrasimplified device that offers rewards and challenges to motivate users and keep them active.

The touch-screen device, about the size of a pack of Tic-Tacs, starts up with a jaunty song and a video with directions, though you hardly need them to operate it. Setup is easy, and from there you wear it or carry it in a pocket to measure your steps when walking, running or climbing stairs. It was the one device tested that did not need to be synced with a computer.

Striiv has a number of ways to keep people engaged. It gives out trophies and points for accomplishments, like your record number of stairs climbed, and points can also be used to play MyLand, a game in which users build and explore an animated world.

Finally, you can choose one of three charities to receive a donation based on your activity (you’ll have to sync to a computer to make this happen). Walk 60,000 steps to immunize a child against polio. Fewer steps help provide clean water in South Africa or save a patch of rain forest.

Striiv does suffer in a few regards. It measures only calories burned. You’ll need a separate app to measure calories consumed, like Livestrong.com‘s MyPlate. And the device recognizes only the motions of walking, running and stair climbing, so it won’t spot an activity like weightlifting or cycling. That can throw off your calorie count.

Fitbit Ultra Wireless Activity Tracker ($100): Fitbit, a monitor about the size of a large nail clipper, has been around for a while, but the Fitbit Ultra brings some new features to the earlier version, which automatically transmits your data within 15 feet of a Fitbit base connected to a computer.

The Ultra adds an altimeter to count stairs climbed. Like the Striiv, it won’t recognize exercise other than walking, running and stair climbing, but you can use a new stopwatch feature to log a block of time that you can later assign to an activity using the Fitbit Web page or the companion iPhone app.

The Ultra will help you track the amount and quality of your sleep, but you have to manually set the stopwatch to do that as well. Tracking sleep is worthwhile because research shows a correlation between adequate sleep and weight loss.

Fitbit has a food log where you can enter what you eat to see how your calorie intake stacks up against calories burned, as well as the percentage of fats, carbohydrates and proteins you’re eating.

BodyMedia Fit Link ($200 plus $7 a month subscription): BodyMedia’s monitors originally were built for researchers like Jakicic. They are worn on the upper arm and they measure heat, moisture and movement. As with other monitors, BodyMedia’s wasn’t good at recognizing exercise other than walking or running (it recorded lifting heavy weights as “moderate” exercise, which it is not).

The BodyMedia Fit needs to be plugged into a computer to log results, which are charted on an easy-to-follow page. It automatically logs calories burned, how long you’ve been active, how long you slept and the quality of your sleep.

The food log, like most, contains a lot of chain restaurant fare. If you make your own meals, expect to spend time entering the ingredients of your recipes to build your own database.

An especially nice feature is the nutrition assessment, which looks at the makeup of your food over three days. It lets you tweak your settings so if you aren’t losing weight when it indicates you should be, it will revise its calorie estimates specifically for you.

Motoactv ($250 8 GB, $300 16GB): Motorola’s new Motoactv is a small touch-sensitive device that looks — and works — a lot like an iPod nano. It was the most sophisticated of the trackers tested, with a built in GPS receiver and the ability to connect to a heart monitor to record a wider range of activities.

The Motoactv gets around the problem of recognizing motion by requiring users to specify what exercise they are engaged in on a touch-screen menu. Then you have to start a timer. If you go from the elliptical machine to a stationary bike, you have to change the setup.

While it records enough information to be a real training tool for runners and cyclists, their gear has to include all of the right sensors to get the full benefit.

One of the features that make Motoactv different from the others is that it plays music as you exercise and recognizes which songs make you work the hardest, building an inspiring “Performance Playlist.” You can also designate a “Fit Song,” so at the touch of a button you hear the tune that always psychs you up to go all out.