Can you really trust your fitness tracker?

Fitness trackers aren’t accurate. There, I said it. I’m sorry if that made you spill your Lucozade over your sweat-wicking T-shirt, but it’s true. It doesn’t matter whether you’re strapping on a Jawbone UP3, Microsoft Band 2, a Fitbit Surge or an Apple Watch, the stats you get from your tracker are an estimate at best. The calories you think you burned yesterday, you didn’t. You probably didn’t take quite as many steps as you think you did either. But before you fire up eBay, here’s another thing to consider – what if the accuracy of your fitness tracker doesn’t really matter? 

Can you really trust your fitness tracker?

The fitness-tracking fad

“The first pedometers hark all the way back to Thomas Jefferson.”

Fitness tracking isn’t new. We’ve had the capability to monitor many of the things that fitness bands track for a long time. The first pedometers hark all the way back to Thomas Jefferson, and the modern step-counting clip-ons that we started getting free in cereal packets in the early 2000s were first seen in Japan back in 1965. Meanwhile, Polar’s wireless heart-rate monitor was being strapped around chests as early as 1982, and we’ve had GPS watches for well over a decade.


So why have we now become so obsessed? One word: smartphones. These mini computers have freed all of our previously stagnant data, allowing it to be packaged up in a more potent, more visual, more interactive and more shareable format. The lure of being in charge of our own data is driving wearable uptake off the scale. ABI Research suggests we’ll see 90 million wearables shipped by 2017, while a recent Juniper Research report predicts that the use of activity trackers – or fitness wearables – will grow threefold by 2018. 

But with so many people now putting their faith into fitness technology, you’d think the accuracy would have to be pretty good. Not quite. The majority of studies so far don’t paint a pretty picture. An Iowa State University study tested the accuracy of the calorie-burn figures for eight popular activity trackers. The results found that most were off by somewhere between 9 and 13% compared to more precise (and vastly more expensive) lab equipment. One tracker was as far out as 23.5%.

So, does this matter? Well, yes. If like an increasing number of fitness-tracker users, you’re using these figures to inform your eating habits, then this could have some quite serious repercussions.

Microsoft Band 2 golf data

A subsequent study – also out of Iowa State University – found even more bad news. The research looked at more than 50 healthy participants aged 18 to 65, with each individual simultaneously wearing a Fitbit Flex, Polar Loop, Misfit Shine, Nike+Fuelband SE, Jawbone UP on the wrist, Actigraph GT3X+ on the waist, and BodyMedia Fit Core on the left arm, while being concurrently monitored with the Oxycon Mobile, a portable metabolic system.

“Four of the activity trackers produced error rates of 15 to 18%.”

While strapped in, subjects were asked to perform 20 minutes of self-selected sedentary activities, 25 minutes of aerobic exercise on a treadmill, and 25 minutes of resistance exercise, with five minutes rest between each activity. Four of the activity trackers produced error rates of 15 to 18%, while two were significantly worse. 

What does this mean for those of us trying to shift a love handle or two? Well, if you’re basing what you eat on calorie-burn figures, you’d be wise to think again. You might not have earned that treat you think you deserve. If you’re feeling good about how active you’ve been, that also might need a second look. The number on your wrist is unlikely to be the whole truth. And I don’t need science to tell me that my tracker’s not quite right when it says I took 150 steps in the shower this morning. 

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