How Accurate is Your Smart Watch?

We see it all the time around the gym. People looking at their watches and discussing how many calories that killer workout just made them burn. Or looking at their watch to check their heart rate while they’re doing cardio, something we are advocates of. Since a few studies have come out taking a look at the accuracy of these devices, we thought this would be a good topic for this episode of the podcast/newsletter.

An estimated 320 million health and wellness wearable devices were shipped in 2022. With so many of these things in circulation, we feel it is helpful to understand what they are accurate at tracking, and what may be a bit of a stretch. After all, they claim to look at many different things such as: steps, heart rate, sleep quality, overall readiness, energy expenditure (calories burned), and heart rate variability.

We begin by looking at a systematic review of 65 studies (linked below) that looked into this question. They looked at a number of different brands such as Apple, Fitbit, Garmin, Polar, Withings, Jawbone, and others. The overall findings were similar to what previous studies had found: these devices seem to be accurate at measuring step count and heart rate, not so accurate at measuring (or rather estimating) energy expenditure. It is likely the fitness tracker may not be taking into account the many individual differences in metabolism from person to person.

The good news is that the use of these activity trackers has been shown to be effective at increasing physical activity in various different age groups and populations. The use of activity trackers increased daily activity by roughly 1800 steps which equates to about 40 minutes of walking, while also favorably affecting body composition. This is a big reason why these devices have become so popular.

On the issue of sleep, there are few studies looking at the accuracy of sleep trackers. So far, research has only shown sleep trackers to be about 78% effective when looking at sleep vs. wakefulness. The accuracy falls to 38% when estimating how long it took someone to fall asleep. These are both compared to polysomnography, which are what sleep experts use when diagnosing sleep disorders. So when you couple this with studies showing the negative effects of knowing you got a bad night of sleep, we are not ready to put much stock in the sleep trackers at this time.

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Simplified Takeaways…

1. Wrist -wearable fitness trackers seem to be accurate at measuring step count and heart rate. Much less accurate at measuring energy expenditure and sleep quality.

2. Wrist-wearable fitness trackers are effective at helping people increase daily activity.

3. Use caution when tracking your sleep with these fitness trackers. The accuracy is a little questionable and the power of suggestion is real when learning you got a poor night sleep. It can set you up for a bad day.

Further Reading…

1.   Systematic Review of Wrist-Wearable activity trackers

2.   Study showing activity trackers boost physical activity

3.   Study showing accuracy of sleep trackers

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