When I read the article, this exact question came out as borne out of genuine curiosity justifying the need for measuring and comparing stuff, not a rhetorical question aiming to dismiss the thing.
And indeed, down the road, the article says:
> “There’s a lot going on between when the heart beats and when the blood gets all the way to the arm or wrist, what’s called pulse arrival time,” Tenan explained. “By simulating that and knowing what’s happening at the heart, we determined what’s being measured with the consumer wearable is different. It’s not to say the consumer wearable is bad or not useful, it’s just not the same.”
As in, electric signal ECG/EKG at chest level and blood flow PPG at finger/wrist level factually are not the same measurement, and asking "do they somehow relate in any way?"
Then the question is about the difference between RMSSD and SDNN/SDRR, and here this article (2022) explains it well:
> “I don’t fundamentally see any reason why any of the wearable companies should be using the RMSSD measure
with:
> RMSSD is considered the best measure for short-term variations of HRV, but still a robust measure for longer-term analyses, with typical use cases for tracking stress, sickness, training, and recovery.
> SDNN is more useful for longer term cardiac health trends and analysis.
And indeed, down the road, the article says:
> “There’s a lot going on between when the heart beats and when the blood gets all the way to the arm or wrist, what’s called pulse arrival time,” Tenan explained. “By simulating that and knowing what’s happening at the heart, we determined what’s being measured with the consumer wearable is different. It’s not to say the consumer wearable is bad or not useful, it’s just not the same.”
As in, electric signal ECG/EKG at chest level and blood flow PPG at finger/wrist level factually are not the same measurement, and asking "do they somehow relate in any way?"
Then the question is about the difference between RMSSD and SDNN/SDRR, and here this article (2022) explains it well:
https://tryterra.co/blog/measuring-hrv-sdnn-and-rmssd-3a9b96...
And answers TFA's question:
> “I don’t fundamentally see any reason why any of the wearable companies should be using the RMSSD measure
with:
> RMSSD is considered the best measure for short-term variations of HRV, but still a robust measure for longer-term analyses, with typical use cases for tracking stress, sickness, training, and recovery.
> SDNN is more useful for longer term cardiac health trends and analysis.