In Part I of this series of The Definitive Guide To Heart Rate Variability For Training, we saw how heart rate variability (HRV) can serve as an indicator of the stress response if measured under certain conditions.
In this blog, we clarify what those conditions are, and get more practical, covering available technologies and protocols for accurate HRV measurement, especially in the context of endurance sports.
Ready? Let’s jump in!
What Is The Difference Between Electrocardiography (ECG) And Photoplethysmography (PPG)?
The first distinction to make is between ECG and PPG. ECG refers to measuring the electrical activity of the heart: this is in fact, the only real HRV.
On the other hand, with PPG we measure changes in blood volume at the periphery (e.g. the wrist or the finger), and therefore we measure pulse rate variability, or PRV, even though we normally still (erroneously) call it HRV.
An example of a sensor relying on ECG is a Polar chest strap, typically one of the best sensors to measure HRV.
In terms of PPG, we have various options, from phone-based apps that do not require sensors (e.g. HRV4Training, which I build) to wearables such as the Oura ring, Garmin watches, or the Whoop band.
Obviously, blood flows when the heart beats, hence ECG-derived HRV and PPG-derived HRV (PRV!) are highly correlated, but they are not the same thing.
The takeaway? Pick a sensor and use it consistently, as there can be small differences in your absolute values when using different technologies, especially if measuring at different locations.
However, both technologies tend to capture the same relative changes over time, which is what really matters when it comes to HRV, as we will see in the next article in this series.
Regardless of the sensor you pick, there are some important considerations to make on the measurement protocol, in particular, related to measurement time and body position.
Let’s look into it in more detail.
Measurement Protocols
HRV is a marker of stress only when measured at rest, far from confounding factors.
Measuring HRV under different conditions will not necessarily be linked to the processes that we want to capture, as many other factors can impact the data (even just drinking water!).
This means we have only two options in terms of measurement protocols if we want to assess our physiological stress level: first thing in the morning or during the night.
Historically and in the scientific literature, HRV measurements were typically taken first thing in the morning: this is the ideal moment for a resting measurement of our physiology, as it happens in a reproducible and consistent way daily.
Under these circumstances, it is relatively easy to avoid confounders (e.g. we measure before exercising, eating, drinking, social interactions, and all other things that might impact the data acutely, hiding our response of interest).
As an alternative, the night has also been used as a moment to track changes in resting physiology.
The night is a bit different, as we still have large changes in HRV on a minute-by-minute basis, for example, due to sleep stages (your HRV during REM sleep and during deep sleep can be dramatically different), but we can get a decent assessment of resting physiology if we average many hours of data.
In the morning, we can get away with a single minute, as we are awake, and do not have to worry about sleep stages or changes in physiology due to the circadian rhythm, which also impacts long recordings, such as the ones obtained in the night.
What Is The Difference Between Morning And Night Data?
There are two main differences between morning and night data, which might play a role in your decision to use one modality or the other.
In particular, the relation between the measurement and the previous days’ stressors differs: the night comes before the morning, and as such, it is closer to e.g. evening stressors.
If you had a heavy dinner, had any alcohol, or exercised late, night HRV will likely be suppressed, not because you need extra recovery, but simply because the measurement was taken too soon.
Night measurements often end up capturing your behavior, not your body’s response to that behavior.
Measuring in the morning on the other hand allows enough time between the stressors and the measurement so that you can capture your response.
If HRV is still suppressed in the morning, then some of the stressors have a long-lasting impact on your physiology, and some adjustments might be required (something for the next article).
The second important difference between morning and night measurements is body position.
When measuring in the night, we don’t really have a choice, and we will be measuring while lying down. On the other hand, in the morning, we can measure HRV either lying down, sitting, or standing.
What’s the difference here?
Measuring while sitting or standing can be more sensitive to stress, allowing you to better capture your response to training (and other non-training-related stressors).
Oftentimes, in physiology, we see more interesting responses when perturbating a system: for example, in a glucose tolerance test, you ingest glucose and see how the body responds. This is more informative than just measuring your resting glucose level before you eat anything.
Similarly for HRV, measuring after you sit up, can be more informative than measuring when you lie down. Sitting up is an orthostatic stressor: your body quickly readjusts to the change in body position.
This process amplifies the stress response, making the measurement more sensitive and therefore more useful.
After sickness, for example, I can often see, over several days, a gradual comeback to normal of my resting heart rate measured while sitting up. On the other hand, my night heart rate goes back to normal faster, unable to capture this recovery phase.
Practical Tips
From the previous sections, you have probably figured out that measuring first thing in the morning, within a short time after sitting up (e.g. 30-60 seconds tops), is what I’d consider the best protocol for HRV measurement, especially for endurance athletes.
I’d stress this as endurance athletes tend to have quite low resting heart rates and train high volumes, often making measurements taken in a highly parasympathetic state (i.e. lying down, and while sleeping), less able to capture their stress response effectively.
In terms of the actual protocol, make sure to go to the bathroom should you need to, then go back to your bedroom, sit up, and take your 1-minute measurement.
During the measurement, breathe naturally, and make sure to avoid things like yawning, swallowing, or movement, as these would cause transitory artifacts and confound the measurement.
That’s all there is really. Repeat every day, and you will start collecting useful data points representative of your response to training and non-training-related stressors.
If a morning measurement does not fit into your routine, a wearable that measures HRV in the night can be a viable alternative, but keep in mind the possible issues described above: night data will likely better reflect your previous behavior than your readiness for the day, and might be less sensitive to various stressors.
This means that you might want to be a bit less reactive when using night data and monitor longer-term trends instead.
Apps And Sensors
For morning measurements, HRV4Training has been validated both with the camera (no sensors required) and in combination with a Polar H7 or H10 strap.
Either method is fine, but make sure to use the same method consistently.
An alternative could be Elite HRV paired with the same Polar strap.
In terms of devices that can capture night data reliably, the Oura ring, Garmin watches, and the Whoop band provide comparable data in terms of night averages.
In the next article in this series, we will look at how to interpret HRV data and what changes we could implement to our training.
