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.

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.

A 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.