Are fitness trackers as accurate as you think?
There are few things more satisfying than dropping on the couch after a run and seeing how many calories you've destroyed. But there's a chance your activity tracker is pandering to your desires.
"Even the best trackers overestimate calories burned by 30 per cent or more," Otago University PhD candidate Leon Mabire said.
So outraged are some fitness bunnies that they've filed law suits in the United States against activity tracker giant Fitbit, claiming the device is "highly inaccurate".
As part of the US lawsuit, the plaintiffs' lawyers commissioned a study that tested Fitbits against ECG heart rate monitor the Zephyr.
The study found devices can be off by up to 20 beats a minute during intensive workouts, and the margin of error increased with intensity.
Fitbit, a company worth US$8 billion, has hit back at the study, saying it was biased, baseless and lacked scientific rigour.
The company's website does not list a percentage of accuracy for its products, but does state: "Like all heart-rate monitoring technologies, accuracy is affected by physiology, location of device, and different movements."
"There isn't a gold standard device that can be accurately compared to Fitbit," the Daily Mail reported.
That is where Mabire comes in. He says he has developed an algorithm that can predict calories burned with an accuracy of 94 per cent.
It took into account body weight, height, body mass index, and a couple of secret, commercially sensitive things, he said.
When he tested the Fitbit Charge HR ($259.95) and the clip-on Fitbit Zip ($99.95) as part of his research, he found their accuracy was between 60 and 75 per cent.
In his sample size of 62, the Charge read 120 per cent of actual calories burned while the Zip served up 135 per cent.
Mabire tested subjects in a range of shapes and sizes, ranging from athletes with a BMI of 19 to the morbidly obese.
"Everyone is different. You don't have a standard Aspirin dose, you have it according to your body weight," he said.
"Why don't we do that for exercise? Exercise is medicine too."
A Berkeley science review found "Fitbit is good for counting steps, might need some help when it comes to measuring distance, and is not particularly reliable for counting calories burned".
A Fitbit spokesman said the company's research team rigorously researched and developed the technology for the Charge HR three years prior to introducing it to market and conducts ongoing internal studies on its products.
"Consumer Reports independently tested the heart rate accuracy of the Charge HR and Surge after the initial lawsuit was filed in January and gave both products an 'excellent' rating," he said.
"We stand behind our heart-rate monitoring technology and all our products, and continue to believe the plaintiffs' allegations do not have any merit."
Several posts on Fitbit's Facebook page question its accuracy.
In a post from Mangatainoka, a woman complained her Fitbit recorded 16,509 steps and 2332 calories burned in a day when she did only 8000 steps, and that it recorded activity while she was sleeping in another room, with her tracker resting on a table.
Fitbit suggested she decrease her tracker's step-counting sensitivity.
Mabire said he did not want to compete with Fitbit. He wanted to create a medical-grade device.
"We'd like to have a prototype by the end of 2016, then we can start testing it, then possibly look at a product in some form next year."
Aside from going for a $5000 AMP Scholarship on June 1, he hopes he can make use of some of the record $2.2b injection into healthcare, announced as part of the Budget on Thursday.
"There was talk on increasing funding in the medical technology area, and something like this would be right up their street."
Do you use a fitness tracker like a fit bit? Do you think after reading this article you’ll be more cautious when analysing the results?
Share your thoughts in the comments below.
Written by Rachel Thomas. First appeared on Stuff.co.nz.
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