Six ways your smartwatch is lying to you, according to science

The Conversation about smartwatches

Topic: Exercise and Fitness tracking

Smartwatches and fitness trackers are not always accurate

You check your smartwatch after a run and see that your fitness score has dropped, you've burnt hardly any calories, and your recovery score is really low, so it tells you to take the next 72 hours off exercise. You've burnt hardly any calories. Your recovery score is really low. It's telling you to take the next 72 hours off exercise. You've burnt hardly any calories. Your recovery score is really low.

The worst bit is that the whole run felt amazing

So why is your watch telling you the opposite

Ultimately, it's because smartwatches and other fitness trackers aren't always accurate

Smartwatches can shape how you exercise

Using wearable fitness technology has been one of the top fitness trends for close to a decade. Millions of people around the world use them daily

These devices shape how people think about health and exercise. For example, they provide data about how many calories you've burnt, how fit you are, how recovered you are after exercise

But your smartwatch doesn't measure most of these metrics directly. Instead, many common metrics are estimates. In other words, they're not as accurate as you might think. But your smartwatch doesn't measure most of these metrics directly

1. Calories burned

Calorie tracking is one of the most popular features on smartwatches. However, the accuracy leaves a lot to be desired

Wearable devices can under- or overestimate energy expenditure by more than 20 per cent. These errors also vary between activities. For example, strength training, cycling and high-intensity interval training can lead to even larger errors

This matters because people often use these numbers to guide how much they eat

For example, if your watch overestimates calories burned, you might think you need to eat more food than you really need, which could result in weight gain

2. Step counts

Step counts are a great way to measure general physical activity, but wearables don't capture them perfectly

Smartwatches can under-count steps by about 10 per cent under normal exercise conditions

For most people, this isn't a major problem, and step counts are still useful for tracking general activity levels

3. Heart rate

Smartwatches estimate your heart rate using sensors that measure changes in blood flow through the veins in your wrist

This method is accurate at rest or low intensities, but gets less accurate as you increase exercise intensity

Arm movement, sweat, skin tone and how tightly you wear the watch can also impact the heart rate measure

This can be problematic for people who use heart rate zones to guide their training

4. Sleep tracking

Almost every smartwatch on the market gives you a sleep score and breaks your night into stages of light, deep and REM sleep

The gold standard for measuring sleep is polysomnography. This is a lab-based test that records brain activity

But smartwatches estimate sleep using movement and heart rate

So even if your watch says you had poor deep sleep, this may not be the case

5. Recovery scores

Most smartwatches track heart rate variability and use this, with your sleep score, to create a readiness or recovery score

Heart rate variability reflects how your body responds to stress

This means most recovery metrics are based on two inaccurate measures

As a result, if your watch says you're not recovered, you might skip training — even if you feel good

6. VO₂max

Most devices estimate your VO₂max — which indicates your maximal fitness

The best way to measure VO₂max involves wearing a mask to analyse the amount of oxygen you breathe in and out

But your watch cannot measure oxygen use

But smartwatches tend to overestimate VO₂max in less active people and underestimate VO₂max in fitter ones

This means the number on your watch may not reflect your true fitness

What should you do?

While the data from your smartwatch is prone to errors, that doesn't mean it is completely worthless

These devices still offer a way to help you track general trends over time

It's also important you pay attention to how you feel, how you perform and how you recover

Hunter Bennett is a lecturer in exercise science at Adelaide University

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