How accurate are smart watches, and are they good for your health?

How accurate are smart watches, and are they good for your health?
  • PublishedJanuary 27, 2024

Dr Richard Alcock’s lungs feel like they’re exploding, but he has to keep pedalling.

A mask is strapped tight to his face to capture every breath he manages to squeeze out over 15 minutes of increasingly intense cycling.

A physiologist is pushing Dr Alcock to his limit: “Treat it like a finish line!”

He manages to respond with only a grunt, and a nod.

A man sweating while cycling, while wearing a mask with tubes coming out.
Cardiologist Richard Alcock testing his VO2 max while cycling.( ABC News: Cason Ho )

He’s testing the maximum amount of oxygen his body can use — his VO2 max — the gold standard indicator of cardiovascular fitness.

All this effort achieved a reading of 62.5 mL/kg/min — it puts Dr Alcock, who works as a cardiologist, close to the Olympic level for his age and gender.

His smart watch shows a reading of 56 mL/kg/min, about 10 per cent off.

Do smart watches give accurate readings?

The difference between the two readings in Dr Alcock’s test is the smart watch hasn’t actually measured his oxygen output.

It estimated the number, based on heart rate and speed.

Smart watches are sold as an easy and accessible way to track your health and fitness, but even the best smart watch falls short of the accuracy required in most medical and scientific settings.

Spanish study published in 2023 found heart rate readings varied significantly depending on exercise intensity, and whether a subject was moving their arms — for example, cycling versus running.

Another study found wearables overestimate sleep because they depend on body movement – but noted accuracy was improving compared to older models.

Smart watches generally struggle to accurately track metrics like blood pressure and the quality of your sleep.

Step count is relatively accurate among most wearables, and can quite reliably measure how far you’ve run under optimal conditions.

However, errors can compound when devices rely on one reading to calculate another, like using your step count to calculate how many calories you’ve burned.

Are smart watches good for your health?

More than 36 per cent of Australians own a smart wrist wearable, according to a 2023 Telsyte market study.

While they have niche medical uses — like detecting an abnormal heart beat, or atrial fibrillation —they’re primarily marketed as general health and fitness trackers.

Dr Alcock believes smart watches help motivate people to be more active.

“Every day use for the general public, they’re great … I think it’s getting people exercising simply by having the watch.”

Many elite level athletes have also adopted them into their arsenal of tools to help maximise performance, according to Edith Cowan University Professor of Human Performance Sophia Nimphius.

“Elite athletes have a very specific training environment, how much training they’re doing, everything is prescribed,” she said.

A woman with a determined look crouched down tying her shoe lace on a bushy outdoor running track.
Edith Cowan University Professor of Human Performance Sophia Nimphius.( ABC News: Cason Ho )

That strict environment is why Professor Nimphius believes smart watches and fitness trackers are more beneficial to athletes, rather than the average user.

“The general consumers’ daily training environment is quite dynamic and ever changing,” she said.

“When we’re thinking about the accuracy of something, a lot of people might think, oh five to 10 per cent [off], that’s pretty good.

“We start taking literal, small changes that might be in that margin of error as if it’s a truth — that’s where we run into some issues.

“Some of the measure and metrics of wearables can be as accurate, or inaccurate, as 50 per cent off.”

A watch with a digital screen on a person's wrist.
Professor Nimphius believes athletes get more benefits from fifitness trackers than the average user does.( ABC News: Cason Ho )

Professor Nimphius suggests relying on trends in your data over time, rather than obsessing over your daily figures.

However, even then, she warns fitness trackers don’t work for everyone as a health and fitness tool.

“Their watch says maybe they’re having a bad day, or they didn’t sleep well, independent on whether they actually feel that’s true,” she said.

“There’s a psychological component to fully trusting and investing in a number on your watch.”

Where does all your data go?

Professor Nimphius uses a smart watch herself, but urged consumers to be aware of how companies are using their data.

“It’s got our heart rate, it’s got sometimes respiratory rate, it’s got when we are sleeping and how much,” she said.

“All of that information when tied together can really say some intimate things about you.”

She said fitness data tracked by wearables, and other technology, doesn’t have the same regulation as other medical or health records, which leaves it open to misuse.

“We have to start considering whether the combination of all these bits of information of our physiology is indeed health information,” she said.


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