Sweat stings my eyes as I pound my way up another step. My ass is seriously dragging; it feels like my shoes are sandbags. How long have I been hurling myself up and down these bleachers? I glance at the two watches on my left wrist. Six minutes have gone by. That can’t possibly be right. I check the watch on my right wrist. Damn. Six minutes. I check my iPhone. Six minutes. Time has slowed to a crawl and, according to all of these gadgets, so have I. My heart rate is a mere 165 beats per minute. I adjust one of the three monitors strapped to my chest and pick up the pace.

This grueling 15-minute bleacher run is the final leg of a training circuit I’ve devised to test seven wearable exercise monitors. I want to get a better idea of how they track calories. The devices range from simple fitness trackers (FitBit Ultra, Nike+ FuelBand, BodyMedia Link) to GPS watches (Motorola MotoACTV, Garmin Forerunner 910XT, Polar RCX3) that communicate wirelessly with sensors pasted all over my body. I’m also testing MapMyFitness on an iPhone 4s linked to a Wahoo Bluetooth heart rate monitor.

All of these gadgets promise to help you get in shape by tracking how much you move and how many calories you expend during a wide range of activities. Motivating couch potatoes and providing everyday athletes with data will be an increasingly lucrative business as so-called “wearable” computing devices like fitness trackers take off. Companies like Nike, Adidas and Motorola are expected to ship 90 million wearables by 2017, according to ABI Research.

Yet as anyone who’s ever used a pedometer, treadmill or heart rate monitor knows, accuracy is relative when it comes to counting calories. This is especially true of wearables, as these gadgets are known. Every device takes a different approach to calculating the mechanics of moving your body a certain distance at a certain speed. For instance, fitness trackers feed step counts and even sweat rate into their equations, while GPS watches lean heavily on heart rate.

To establish a level playing field in benchmarking these gadgets, I need to do each test with every device strapped to my body. I’m wearing three heart monitor chest straps on my torso, a temperature sensor on my arm, three GPS receivers on my wrists and four motion sensors on my arms and legs. I look like I just escaped from a lab.

Before hitting the bleachers I ran 5 kilometers. This followed the 50-minute plyometric workout and 2-mile walk I did the previous day. I endured the same routine three times in two weeks. For each activity, the calorie counts reported by these exercise monitors were all over the map. That’s to be expected, says Dan Heil, an exercise physiology professor at Montana State University.

“Everyone assumes when a device gives a calorie count that it’s accurate, and therein lies the danger,” he says. Just because the watch on your wrist says you burned 1,000 calories, it isn’t necessarily true. "There’s a huge margin of error and the true calorie burn lies somewhere between 600 and 1,500 calories."

Based on his experience analyzing athletes at the university’s Human Performance Lab, Heil has found calorie counts will be inaccurate unless you’re working in a controlled environment. That’s because calories aren’t so much tracked as estimated using sophisticated (and proprietary) algorithms. These algorithms attempt to discern your rate of caloric expenditure using measureable or self-reported metrics like heart rate, distance, weight and age.

Each device I tested crunches the numbers differently. Wahoo Fitness, for instance, calculates burn rate in its iPhone app using the same two algorithms for every activity. The formula for women is [(-20.4022 + (.4472 x heart rate) + (.278 x weight) + (.074 x age)] / 4.184]. For men it is (-55.0969 + (.6309 x heart rate) + (.438 x weight) + (.2017 x age)] / 4.184.

Everyone else declined to reveal their algorithms, citing the usual pending patents and trade secrets. But some shared the metrics they used in the equation, which helps explain the results of my testing (.xlxs).

My running test had by far the most consistent numbers. The only outlier was the MotoACTV watch, which didn’t record more than 298 calories for the same workouts that saw the rest of the trackers report an average of 452 calories. I expected this sort of uniformity from the watches because their calorie equations rely heavily on heart rate, but I didn’t think the accelerometer-based Fitbit, FuelBand, and BodyMedia Link would match up as closely as they did. I mistakenly thought their approach of counting steps and overall movement was geared toward less-intensive activities like walking and yoga. This test shows that casual runners who don’t want to wear heart rate straps can get by with a fitness tracker instead of a GPS watch if they’re only interested in counting calories.

Oddly enough, when it came to the least intense of my four routines, walking, the Fitbit delivered some unusual results. It claimed that I burned almost as many calories walking for 35 minutes as I did running for 25. I thought this might be due to the fact I wore the Fitbit on my waist and the other fitness trackers on my right arm, since each swing is often interpreted as an extra step. I discovered it was due to one of the FitBit’s default values. The Fitbit automatically calculates separate values for running and walking stride lengths based solely on your height. In my case these default values were off by about three inches for my running stride length, and this significantly threw off the caloric calculations.

I asked Heil why it is so hard to build a good algorithm based solely on movement. He cited two reasons. First, if the device doesn’t take into account what’s happening inside your body — heart rate, skin temperature, breathing — then there’s nothing specific to your particular physiology in the calorie burn algorithm. That’s why everyone who enters the same body weight on a treadmill will get roughly the same numbers after a three-mile run, regardless of their level of exertion. The second reason is because these devices, for the most part, have difficulty determining exactly you are doing as you’re doing it.

“A GPS watch for example, records changes in your position in three dimensional space and then calculates your speed,” said Heil. “But it can’t tell if you’re skipping or walking backward, and each action requires a different level of energy expenditure.”

Correctly identifying the activity means knowing how much muscle mass you use — more mass equals more calories. Sports such as cross-country skiing and swimming burn more calories than ping pong. Accelerometer-based devices have the same problem, though some do a better job of recognizing activities than others. The Nike FuelBand, most notably, maps the movement of your wrist across three dimensions and compares those movements to familiar exercise patterns, such as jumping jacks, tennis and handball. The BodyMedia Link, on the other hand, matches overall movement to changes in your skin’s surface temperature, or more appropriately, your body’s reaction to training intensity.

According to Heil, temperature, by its nature, delivers a delayed reaction, so if you go from a slow jog to a fast run to a walk your skin temperature doesn’t adjust quickly with your intensity. This can lead to a lower calorie count for vigorous activities. This explains how the Link’s scores in the bleacher run and polymetric tests fell short of those posted by the Garmin and Polar watches. Of course, the Link awarded more calories for the walk, which may explain why its broad stroke approach works when compiling everything you’ve done into a complete picture of your caloric expenditure throughout the day.

Like all fitness trackers, both the FuelBand and the Link are better suited (and in fact recommended) for getting a bead on how many calories you burn during the day instead of during a specific workout. They serve as an incentive to get off your ass and get moving.

Although the MotoACTV also sports an accelerometer, it isn’t as accurate as that of the FuelBand and doesn’t use it to identify different movements. It’s only there to step in if GPS is out to lunch. For example, if you lose the satellite signal midway through a run, the watch can still track your progress using the accelerometer and even tap an onboard altimeter to determine whether you’re going uphill and therefore burning more calories.

The MotoACTV doesn’t bother trying to figure out what activity you’re doing; it only has you select from among 46 activity modes. (Bleacher running isn’t on the list.) To determine your calorie burn, the watch uses a different multiplier for each of the 46 activities supported by the watch. The multipliers, which Motorola refused to divulge, can’t be too big because the watch wasn’t terribly generous with the calorie counts, even during my toughest workouts. In all but my walking test, the results it offered were much lower than the others. Magno Herran, product manager at Motorola Mobility, says many fitness tracking gadgets are overly generous when it comes to awarding calories. He also recommends using the MotoACTV with a heart rate monitor to ensure the most accurate calorie counts.

He’s got a point. There was a clear divide between movement- and heart-rate-based calorie algorithms in my most physically demanding tests, especially plyometric workout. That’s where the Polar RSX3 and Garmin Forerunner 910XT stood out against the other gadgets.

The two watches rely heavily on heart rate to crunch the numbers. For instance, the Polar factors in maximum heart rate, current heart rate, sitting heartrate and an estimated VO 2 max in its algorithm, along with weight, height, age, and gender. This may explain why both watches awarded nearly the same number of calories for every activity despite the fact that the Forerunner logged my movement with GPS and the RCX3 tracked it with an accelerometer attached to my shoe.

This was most apparent during the plyometrics, which are notoriously high-effort drills like jump squats and lunges in which your heart rate spikes with each interval set. The series of explosive movements seriously juiced the calorie count on the two watches, but the rest of the devices spat out more mundane numbers tied to my movement and the limited number of steps I took. The outlier in this case was the MapMyRun app. Since it was linked to a heart rate monitor, the app should have reported the same high numbers as the Garmin and Polar watches, but instead delivered the lowest calorie count of all the devices. This is likely due to how little weight the app's calorie algorithm puts into the heart rate numbers.

In the final analysis, these exercise monitors provide calorie numbers that are good ballpark figures, but not something you can completely rely on. To get that, you need an indirect calorimeter device, which most commonly comes in the form of a 5-pound backpack-sized contraption that analyzes your oxygen consumption using a face mask or mouthpiece. Heil says indirect calorimeters are the best way to track calorie burn since breathing has a direct relation to the amount of energy you use. Unfortunately, calorimeters cost $30,000 to $50,000 and are rarely seen outside of labs.

Even if they aren't completely accurate when it comes to calories, fitness gadgets like these still give you a bird's-eye view of how active you are. The best of them, like the Polar RCX3, Motorola MotoACTV and Garmin Forerunner 910XT, even allow you to drill down into the particular metrics of each workout and track your progress over time to see how you're improving. This may not make your bleacher runs any less of a grind, but it may help you get fitter faster.