When we where looking for scientific smartwatch accuracy tests at the highest level, for instance with 3 channel EEG devices which are gold standard for runs on a treadmill up to to around 15 km/h:

The Quantified Scientist Rob ter Horst started testing scientifically for instance smartwatches from Apple and Google against the Polar H10 chest strap:

But Rob delivers no numical and statistical data at the level of his qualification which would be worth publishing in a high level scientific journal. If you want to know more about this topic, check Peter Gamma’s site www.petergamma.org:

tag «PPG»:

https://petergamma.org/category/optical-heart-rate-sensor

There you can find 370 post about PPG devices. Many where trigged by Rob ter Horst, since he delivered us the opposite of what we where looking for.