The first version of the Apple Watch, launched a decade ago, revolutionized the wearables market for fitness enthusiasts. Using a built-in heart-rate sensor, accelerometer and gyroscope, the wrist-worn device allowed users to monitor their fitness stats in real time. But that wasn’t the biggest bonus of the Apple Watch. It was one of the first activity trackers worn all day, allowing an unprecedented collection of data on movement patterns, calories burned and physical activity over days, weeks and months. So instead of capturing individual workouts stats like previous activity trackers, the Apple Watch’s always-on feature gave one of the first objective looks at how active you are outside of deliberate exercise.
Other smartwatches like Fitbit, Garmin, Google and Polar have followed, each with its own proprietary set of features geared to fitness and activity tracking, making the wearables market one of the leading growth industries among the active crowd with worldwide sales in 2024 estimated to be US$78.40 billion.
Much of the popularity of smartwatches is due to the ever-expanding list of features that target the super fit and weekend warrior alike. The latest Apple Watch (Series 11) has expanded the number of trackable activities to include a vast array of recreational, sports and wellness activities. It’s also water resistant to six metres, offers water depth and temperature readings, fall detection, emergency SOS and is compatible with an increasingly long list of third-party fitness and wellness apps. But it’s the development of smartwatches from fitness trackers to health trackers that has expanded its appeal to more than just exercise enthusiasts tracking their runs, bike workouts and swims.
Sleep and menstrual cycle tracking, ECG and blood oxygen saturation, low and high heart rate, sleep apnea and arterial fibrillation (AFib) notifications and blood pressure notifications set to debut any day now, smartwatches are now considered a tool for managing health, fitness and wellness.
But how well do these much-advertised features work?
At the most basic level, smartwatches have proved to be pretty accurate at tracking step counts and heart rate. And with more sophisticated sensors in newer versions of the watch, the ability to measure and track a variety of exercise-related metrics like VO2 max, resting heart rate, heart-rate variability, stride length and respiratory rate has improved. Even newer health features, like AFib notifications and ECG readings, have been independently confirmed to deliver a high rate of reliability. But where most smartwatches struggle is in estimating energy expenditure.
The exact formulas used by smartwatch brands to determine calorie burn are considered proprietary, but in general they factor the height, weight, sex and age of the user with the heart-rate response during a bout of exercise. Yet tests done comparing calorie burn measured by a smartwatch and in a lab-based setting with trained staff and sophisticated equipment (considered the gold standard) revealed a large discrepancy, especially as exercise intensity increased.
The reason for the gap between lab-based and smartwatch-based results is multifactorial, but it’s largely due to fact that the formula used by smartwatches to determine energy expenditure is based on large population data sets, which may or may not be indicative of the individual wearer of the watch or the activity being measured. The more outside of the norm the individual or the activity, the greater the margin of error. Indeed, studies have indicated that most smartwatches underestimate the energy expenditure of obese individuals and overestimate that of the very fit.
That said, studies of smartwatches usually lag well behind the most current model, meaning that by the time a study reveals its results, there’s a good chance a more up-to-date version of smartwatch has fine-tuned its algorithm to be more accurate. And most of the newer models of smartwatches gather an increasingly large number of physiological markers that, when combined and analyzed, make the results more personalized and accurate.
If you’re a recreational athlete who uses a smartwatch to monitor performance, progress, activity levels and other basic exercise and health-related data, small discrepancies in results probably won’t make a difference to the enjoyment of your watch. But it’s a reminder that wrist-worn devices have limitations, especially if the sensors aren’t reading your daily metrics due to not being worn snug enough to the wrist or the watch is worn intermittently. Most smartwatch manufacturers state that it takes a few days of continuous wear to gather enough data to create a baseline from which to evaluate fitness, sleep and health.
What a smartwatch does better than any test in the lab, however, is offer a superior view of your health and fitness over time. Smartwatches gather an unprecedented wealth of individualized health and fitness data by virtue of being worn 24/7, so their results reflect your most current health and fitness status as well as how it has changed over time. Lab test results, on the other hand, are a snapshot in time that, due to their expense and lack of accessibility, aren’t often repeated and verified.
There are lots of reasons to wear a smartwatch, and users have continually been shown to increase their physical activity and reduce sedentary behaviours in the months after acquiring a watch. Just be clear that while most of the information it provides is accurate, there’s always a chance that you are an outlier in any one of the physiological measures it tracks and analyzes. Wear your smartwatch in good health and enjoy what it has to offer, just don’t consider it infallible.