Swallowing saliva and real-time heart rate variability (HRV)
There is plenty of hype around measuring HRV all the time.
Despite having developed tools for HRV analysis years before any wearable even existed, we don’t do “real-time stress estimation” at HRV4Training.
Why is that?
When monitoring HRV, and measuring intentionally (e.g. first thing in the morning, as an assessment of our resting physiology), we control what we can to ensure the measurement is representative of what we are interested in: parasympathetic modulation as a proxy of the stress response.
Among what we control, there's swallowing saliva: swallowing temporarily increases heart rate, hence it is something not to do when you take your measurement, as it would cause a very large difference in HRV. This is obvious to anyone who has actually looked at some data: the RR intervals time-series changes, there's an abrupt suppression for a few seconds, the regular rhythm that is normally mostly mediated by your breathing is disrupted, and as a result, HRV suddenly becomes quite high and more variable over time.
This has - of course - nothing to do with "stress", it is an artifact. Now consider that a healthy human will swallow spontaneously 18-400 times an hour - it is hardly possible to find a single minute of HRV data that is collected that is not just an artifact, because of swallowing.
Protocols were not invented to make our lives more complex. We use protocols because at times, they are the only way to meaningfully interpret collected data.
Below is some data where you can see the impact of swallowing saliva on RR intervals (beat-to-beat data used to compute heart rate variability), and on heart rate variability (the rMSSD feature) itself:
HRV when swallowing saliva is twice what it really was, at rest, when forcing myself not to swallow saliva:
actual HRV: ~45 ms
HRV when swallowing saliva: 80 ms
A "real-time stress monitor" would, of course, detect lower stress or relaxation or whatever you want to call it, in the minutes in which my HRV doubles. The relationship between heart rate variability and stress is valid only under certain circumstances: outside of those circumstances, HRV is impacted by many other factors that have nothing to do with how stressed you are. Similar artifacts happen when you simply drink water: also, nothing to do with stress.
Below is an example from published literature, showing the same, i.e. a much higher heart rate variability (in this case quantified as RSA), with swallowing (full text of the paper here):
Use good tools and good protocols, look at the actual data (never at “stress estimates”), and you will be able to use HRV more meaningfully.
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 has published more than 50 papers and patents at the intersection between physiology, health, technology, and human performance.
He is co-founder of HRV4Training, advisor at Oura, guest lecturer at VU Amsterdam, and editor for IEEE Pervasive Computing Magazine. He loves running.
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