Are Fitness Trackers Accurate At Calorie Counting?

It’s hard to walk into a gym these days and see someone without a FitBit or Apple Watch strapped to their wrist. Gone are the days of poorly designed pedometers, the technology these days is hardcore. We can monitor our heart rate, count our steps and apparently, measure the calories we burn.

We all trust our glorified watches to be accurate, however scientists have found that, while the devices are awesome at monitoring heart rates, they are pretty poor at measuring calories. The results, which were published in the Journal of Personalised Medicine tested seven devices, the Apple Watch, Basis Peak, Fitbit Surge, Microsoft Band, Mio Alpha 2, PulseOn and Samsung Gear S2. Participants consisted of 31 women and 29 men who wore multiple devices at once while using the treadmill, exercise bikes and sitting.

“There were diversity of ages, male and female, and then also we looked at diversity of skin tone, and then size and weight to try and represent the population generally,” said co-author of the research, Euan Ashley.

When measuring heart rate, the Apple Watch came out on top with a median error rate of 2%. Samsung Gear S2 came bottom on the pile with a worse but still impressive 6.8% median heart rate error.

However, the devices weren’t as impressive when measuring calories burned. The FitBit Surge had a 27.4% error rating, which was surprisingly the best of the group. Conversely, the PulseOn device had a staggering 92.6% error rate. Not great news if you’ve been relying on your wrist device to help you out at the gym.

So how can these expensive bits of tech get it so wrong? Ashley says it could be down to a range of factors such as poor data input by users or the devices’ algorithms. The errors varied depending on sex and mode of exercise.

We have to take the results from our fitness watches with a pinch of salt. They are a great aid to get to know your body and what it’s capable of, but we shouldn’t be exclusively relying on them.

Is there a reliable way to measure our energy expenditure?

METS, AKA metabolic equivalent of task are a physiological measure expressing the calories burned during exercises. One MET is equivalent to a person sitting or resting. When you exercise, the MET equivalent is compared to rest therefore, the MET value indicates the intensity of the exercise.

You can find the MET value of an exercise on most cardio machines in the gym, like the treadmill, elliptical and exercise bike and there are plenty of websites that show how many METs most activities take up. So how can you work out the number of calories burned? A simple equation:

_____MET Value  x  3.5  x  _____kg body weight  ÷  200 = calories burned per minute.

Note that 3.5 and 200 are both constants and should not be changed.

Example: For a 120 pound person (54.4 kg) doing brisk walking (approximately 4 mph) for 90 minutes per week at 5 METs the total calories burned would be 428.

5 x 3.5 x 54.5 ÷ 200 = 4.76

4.76 calories x 90 minutes per week = 428.4

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