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Wearable fitness tracking devices have been available to the consumer in various forms since the 1980s. The earliest versions were heart rate monitors strapped on runners and connected to treadmills.  We’ve come a long way since then with today’s devices tracking steps, your location and route, calories burned, heart rate, and even hours of sleep you got.  These watch-sized devices can record all your data and display it on your smartphone with some even reminding you to get up and move or go to the gym. 

It seems more and more wearable fitness tracking devices come on the market each year.  It has become a huge industry with companies clamoring to develop features and functions that set them apart from the others.  But how accurate are these devices and is the data they provide useful? 

WearableZone compiled the results of several studies on fitness tracker accuracy in 2017.  Here is what they found, broken down by category of data measured.

Heart Rate.  The medical standard is the electrocardiogram, which measures electrical activity in the heart.  Fitness trackers determine heart rate by shining a light through he skin and estimating your pulse based on how much light bounces back.  The fitness trackers in the studies fared well in their measurements when compared to medical-grade equipment, showing 83-99% accuracy.

GPS.  Once only available to the military, hyper-accurate GPS measurements are now available to everyone.  The same technology that lets us send a missile through the window of a terrorist’s mud hut from halfway across the globe allows nearly every wearable tech device to accurately tell you your route and distance.  There’s even GPS art now, with runners using their fitness trackers to draw pictures with their run routes.  

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Step Counting.  While the most accurate step measurement is a researcher following you around clicking a manual counter, fitness trackers approximate a reasonable facsimile of this with gyroscopes and accelerometers.  The studies found the devices’ estimation of steps taken to be in the 80-99% accurate range.

Sleep.  Medical-grade analysis of your duration and quality of sleep involves a whole bunch of sensors stuck to various places on your body.  They measure respiration, leg twitching, eye movement, and brainwaves. While wearable tech devices have improved in the area of tracking the movement or lack thereof during sleep, these fitness trackers still can’t determine what sleep stage you are in. So, the quality of sleep they are determining is not an actual measurement but an assessment derived from other measured body functions.  The only way to get a full diagnostic reading on your quality of sleep is to go through a sleep study.

Calories burned.  Lab testing of athletes’ calories consumed is done through ergospirometry, where a mask strapped to the face captures exhaled air and an analysis of the composition of this air indicates burned calories.  

A wearable device uses your Basal Metabolic Rate (BMR), which is comprised of data you input, such as your age, gender, height, and weight to estimate your daily caloric expenditure in everyday activities.  It then takes things like the steps measured by its accelerometer and other data you enter like your workouts or other activity to estimate the calories burned.  Everyone’s body is different and it’s impossible to make an accurate generalization of calories burned in a specific activity from person to person.  Add in vague categories each user determines and selects such as vigorous versus moderate running, and you can easily see how the device’s readings can be way off.  

Devices in the studies were all over the place when compared to laboratory device readings, with the best devices showing 87% accuracy and the worst having only 8% accuracy.  When it comes to calories burned, your wearable device cannot be relied upon to show accurate data.

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Wearable tech devices have their place in some fitness applications.  I know people who are more active since owning a fitness tracker because they needed that reminder to get moving or use it to track a goal they’ve set for themselves.  In those situations, yes, definitely use your tracker.  But if you are making diet or health decisions like “Can I have another slice of pizza?” or deciding not to get tested for sleep apnea because of what your watch told you, you may be taking a grave risk trusting that information.

If you use fitness tracking devices, be aware of their limitations, and understand what information provided by them is an accurate measurement and what is at best a wild guess.