Hi there š
I hope you are doing well.
A recent study, led by Eiko Fried, put the application of HRV-based continuous stress monitoring as proposed by wearables to the test, and found that the correlation between wearable data and self-reported stress scores was ābasically zeroā (in the authorās words, according to The Guardian).
Continuous stress monitoring was the nail in the coffin for me when it comes to wearables, as the misuse of HRV data for this application was something I couldnāt get behind.
When assessing in detail 1) the physiology of what we are measuring with HRV 2) the technology used 3) the interpretability of the data, it is clear that there is no such thing as continuous stress monitoring using HRV, and these applications are a wild extrapolation merely serving user engagement.
This is something I discussed in detail here, in case youāve missed it and would like to learn more.
In another recent study researchers looked at self-reported stress measures (collected via a validated questionnaire, similarly to the study discussed above), in relation to physiological measurements (e.g. resting heart rate and HRV measured during the night, not continuously) and made-up scores (the recovery score provided by a wearable).
As night data provides a good assessment of an individualās resting physiology and stress response - albeit not as good as a morning measurement (see here) - a negative correlation between self-reported stress measures and HRV was reported, i.e., the higher the stress, the lower the night HRV.
Similarly, we had a positive correlation between resting heart rate and HRV; hence, the higher the stress, the higher the resting heart rate. The relationship between HRV and stress was also somewhat stronger than the relationship between heart rate and stress, highlighting once again how HRV is probably a more sensitive marker of stress, something I cover in detail in my breakdown of the differences between heart rate and HRV, which you can find here.
Where am I going with this?
The study also reports that there was zero correlation between the made-up recovery score of the wearable and all other variables.
I receive quite a few emails from people concerned with their āstress monitorsā and made-up scores (readiness, recovery, etc.) - and yet none of these parameters are good at tracking what they claim to be tracking.
Keep that in mind before you invest your mental energies and trigger additional worries and concerns for you or your athletes because of made-up data with no physiological meaning.
There is a use for HRV data, but that use is not continuous stress monitoring (quite the opposite!) or building made-up metrics that pretend to capture our overall state.
Cheers!
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Marco holds a PhD cum laude in applied machine learning, a M.Sc. cum laude in computer science engineering, and a M.Sc. cum laude in human movement sciences and high-performance coaching. He is a certified Ultrarunning Coach.
Marco has published more than 50 papers and patents at the intersection between physiology, health, technology, and human performance.
He is co-founder of HRV4Training, Endurance Coach at Destination Unknown, advisor at Oura, guest lecturer at VU Amsterdam, and editor for IEEE Pervasive Computing Magazine. He loves running.
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Thank you Marco for another objective and interesting read.