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Today, we’re delving into the world of activity trackers and how reliable they are at measuring the calories burned from exercise.


As we know, the amount of calories you consume can dictate a whole host of outcomes, from fat loss, muscle gain, performance all the way through to immune function, adaptation and optimal recovery.

Apps such as My Fitness Pal are a great tool for ESTIMATING calorie intake. But, when we start comparing the calories consumed against the calories burnt through training and daily activity, how do they stack up…i.e. can we rely on the data they give us?

The first thing we need to consider is that everything we track is an estimate, it’s used to point us in the right direction as they’ll provide us with a ball park figure to work from.
In a 2017 study by Price et al, they looked at the accuracy of 2 different devices (Garmin Vivofit & Fitbit One) across a range of walking and running speeds. As expected, there were differences in calories burned for every watch at every walking and running speed.

When compared to the laboratory standard method (indirect calorimetry), the Fitbit One was prone to overestimation by 21%, where the Garmin vivofit was prone to underestimation of 27%. That being said (on a whole), Fitbits have a tendency to overestimate calorie expenditure…which is somewhat contradicting to this study.

When you apply this to a real world scenario, someone who does 5 x 30 min walks + 3 x 30 min runs per week can be reported as either 2504kcal burnt (Fitbit One) or 1498kcal burnt (Garmin Vivofit)…where they in fact would burn approx. 1804kcal in total.

As you can see, this difference is pretty huge and not reliable if you’re using them to compare against the amount of calories consumed.

That being said, newer technologies installed in watches now take into account a combination of physiological measures such as heart rate, skin temperature and galvanic skin response which may improve their accuracy. I.e. a £50-£70 activity tracker (used in the study) Vs a £500 activity tracker will ‘most likely’ have different levels of accuracy.

Research (and anecdotally), more expensive smart watch record all measures relatively good at lower intensities, but lose their reliability when intensity builds. To counteract this, the use of heart rate chest straps could be a viable mean to improve the level of accuracy.

Even with this, it’s still an estimate.

Therefore, use activity trackers to point you in the right direction and modify behaviours. If you’re looking for exact and definitive figures, you’ll be pretty disappointed.

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