How accurate are fitness trackers?

Fitness trackers have surged in popularity in recent years. There are several leading, big-name brands, such as Garmin, Fitbit, Samsung & the Apple Watch, as well as many lesser-known brands. Some will just track your steps, while others will measure your heart rate, maybe even use GPS to accurately determine your location and some even measure your sleep patterns.

Fitness trackers evolved from pedometers (step-trackers) as a way to measure activity level. The 10,000 step figure was actually derived by a Japanese pedometer maker, looking to boost sales and while a decent activity goal, has no formal, scientific basis.

One of the first questions then is, should we count our steps? Well, it’s a reasonable measure of activity, however, it’s not the be-all and end-all of how active you should be. If you drive to work, sit at an desk all day and don’t move around much, then having a step goal is certainly something to aim for. If you cycle or walk to work and have a job where you are on your feet all day, you’ll probably hit 10,000 steps before lunchtime.

The main issue with step counting, is it doesn’t factor in how energetic you are. Running 10,000 steps is a lot better for your cardio vascular system, than a slow walk. That said, if you’re not even racking up 5,000 steps in a day, you’re barely moving and that’s not healthy in the long-term. There’s also the danger of double-counting, by adding in your step count to your daily activity, in order to give yourself more exercise calories, when your calorie tracking!

Mid and high-end calorie trackers have heart rate sensors built into them, enabling you to track your exertion level and this will often also give you a figure for how many calories you have expended. There are a couple of problems with this. Some devices are far better at measuring your heart rate than others. Those worn on the wrist move around a lot in exercise so can often misread your heart rate. Some heart rate monitors also only take readings once or twice a minute, which is far from perfect. (The best ones measure your heart rate every second)

The best heart rate monitors are generally those worn on a chest strap, which measure the heart’s electrical signals, but again, some brands are better than others. Polar is one of the leading brands in this category, having been producing chest-strap monitors since 1977 as training aids to the Finnish National Cross Country Ski team.

Measuring heart rate is relatively simple, calculating the calories burned, however, is another matter entirely. How many calories you burn is determined by your age, weight, sex, how much muscle you have, your base metabolism, etc. The only accurate way to measure calorie burn is to wear a mask and measure your oxygen consumption and CO2 production during exercise, something that’s only going to happen in a lab. So, all these fitness trackers use an algorithm to make a best guess, based on certain factors such as your age, weight and sex.

What this means is that your fitness tracker could be over-estimating how many calories you have expended, it could even be under-estimating. I’ve seen articles which claim wrist-worn trackers can be as much as 30% inaccurate (as much as 93% in one case!), however, frustratingly, they don’t explicitly say if it’s over or under-calculating the calorie burn and without knowing how the trackers calculate your calorie burn, we’ll never know. However, given the language used, it’s fair to assume that most of these devices are over-estimating the amount of calories consumed in exercise.

I use a Polar heart-rate monitor to measure my exercise. I also have a Samsung Watch, but I find the heart-rate tracking on it verging on useless. I use my Polar more for a live read out of my heart rate as a determination of how hard I’m working and how fast I recover. The calorie readout for me is a guide, but I certainly don’t take it as gospel. When it comes to calorie tracking, I will only eat some of my exercise calories, usually less than half, unless I feel I really need to have extra. See my other blog post on whether you should eat your exercise calories.

In summary, fitness trackers are a good way to see how active you are and how hard you have worked, but don’t rely on them to give you an accurate calorie figure for the exercise you have done.

Questions? Comments? Let me know below!