Q&A: Should I measure my HRV before training?
A common question when getting started with HRV measurements is whether one should be measuring before training instead of first thing in the morning. The rationale is that we are about to train, and therefore there is no better time to capture our “readiness to train” than that moment.
While the reasoning makes intuitive sense and would be correct if we were able to “capture our readiness to perform anytime”, this is not what we can do with HRV. It’s a limitation of the tool (and the physiology).
Let’s see why and how we can make appropriate use of the technology given the limitations.
Why do we measure first thing in the morning (or in the night)?
HRV provides an assessment of our resting physiology as modulated by the autonomic nervous system in response to various stressors only under certain circumstances.
These “circumstances” are measurements taken 1) at rest and 2) far from stressors (and artifacts!), which means we can capture our “readiness to perform” (or more accurately, our “readiness to assimilate additional stress”) either first thing in the morning or during the night (the latter option being valid only when your evenings are very uneventful). This is also why I recommend not to waste your time with continuous HRV measurements, as I cover in more detail here.
A common misconception is that HRV is a measurement of stress. However, HRV is not a universal measurement of stress, and heart rhythm (and therefore HRV) can be impacted by a number of factors that have nothing to do with stress (drinking water, swallowing, yawning, digesting, talking, breathing, etc.) - only when we control all of these factors - and some more - we can say that HRV is a decent proxy of stress (i.e. only when we measure first thing in the morning, using a good protocol, or during the night after an uneventful evening).
What if we face big stressors during the day, before training?
The question above is a typical follow-up, which further motivates the reasoning behind a measurement just before training.
There are situations in which our ability to perform late in the day is compromised no matter the morning (or night) physiology. A simple case would be e.g. food poisoning at lunch, or other forms of sickness later developed. The issue however is not that your morning or night data might not reflect your ability to assimilate additional stress in case of large stressors during the day, but the impossibility to measure your state reliably outside of those resting conditions - far from stressors - such as the morning and the night.
As covered above, during the day, your diet (and many other factors) will probably drive much of the variability in HRV. For example, if I have a heavy dinner my HRV will stay in the 30s-40s for hours, while if I have a light one it will be in the 60s. None of this means anything, it's just digestion, and maybe a larger meal is better for me if I'm preparing for a big or long effort.
The same applies in the case of stimulants, e.g. caffeine, which would lower your HRV, increase your heart rate, AND get you more “ready to perform”.
See what I mean?
When we measure during the day, we are not able to assess anything meaningful in terms of actionability because there are just too many artifacts and stressors that would be easily misinterpreted (like food intake, caffeine, etc.).
A lower HRV is not necessarily bad but that's always the simplistic interpretation we end up making when looking at continuous data.
Practical implications for your decision making
How do we use the data then, and how do we take into account the impact of stress during the day?
In my opinion, the best way to use the data is take morning measurements as an overall assessment of our physiological state, and then if we train much later in the day, simply to use our subjective feel to make the call.
Is your physiology normal and do you feel good? Great, proceed as planned.
If your physiology is suppressed in the morning, then that day is unlikely to be a good day for additional stress.
If your physiology is normal but you don't feel like a hard session because of the day you had, then it is totally fine to reduce intensity based on your subjective feel.
Morning (or night) data gives you a starting point, but the adjustment later in the day is more meaningful at that point if it is based on how you feel, not on any (HRV) data that is of very little meaning, in terms of your ability to assimilate additional stress.
The only reason to measure before exercise would be to compare with data collected after exercise as an assessment of exercise intensity, but in this context, the pre-exercise data is never used to guide training, but only as a reference point for the post-exercise data. You can learn more about this application, here.
I hope this helps, and thank you for reading.
See also:
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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