What is Heart Rate Variability (HRV) ?

Heart Rate Variability (HRV) is the measure of the variation between each heartbeat. Realised in the 19th century, HRV has been a dormant metric in the world of wellness until relatively recently. If you’re using tech that gives you a readiness or recovery score, chances are it is tapping into your HRV.

But what is HRV?

What is HRV?

If your heart beat is 60 BPM, that does not mean that your heart beats every second on the dot for a minute. Sometimes there will be a 0.9 second gap between beats and sometimes there will be a 1.1 second gap.

That variation between gaps is referred to as HRV, but what causes the variation?

This variation is regulated primarily by a part of our nervous system called the autonomic nervous system. The autonomic nervous system is a combination of our ‘fight or flight’ response (sympathetic nervous system) and our ‘rest & digest’ response (parasympathetic nervous system).

The autonomic nervous system regulates 1000s of processes in your body, including heart rate, and is heavily influenced by a variety of different factors including sleep, menstrual cycle, illness and exercise.

What is a ‘normal’ HRV?

The quick answer to this question is... it depends... on many things. Your ‘normal’ HRV is influenced by:

1. What is measuring your HRV

2. How your HRV is being measured.

3. When your HRV was measured.

4. How long your HRV was being measured.

5. You.

When it comes to you, your baseline HRV is determined by fixed factors like:

1. Age

2. Ethnicity

3. Gender

4. Genetics

So if you have been measuring your HRV for a period of time, ideally over 30 days, and if your average HRV is in and around 50ms, then this is most likely your baseline, provided there has not been massive fluctuations in your life over this period. However, you may have noticed changes to your baseline from day to day depending on what is going on in your life.

Now let’s look at these changes.

Changes to your HRV

Your body works hard to keep itself running efficiently. It does this by making constant adjustments so that it can react to the stressors of your life. Examples of this include shivering when you feel cold and increasing your heart rate when you exercise to provide oxygen to your muscles, but the stressors and adjustments don’t always need to be so obvious.

This is where HRV can come in. HRV is a good proxy into how well your body is adapting to your stressors. If you’ve been exercising more than usual, you may see a decrease in your HRV because your autonomic nervous system is coping with your recovery which is reflected by a decrease in parasympathetic activity dominance and a decrease in your HRV.

It’s important to note that HRV does not reflect how big the stressor is, but rather how your body adapts to it. So if you found you’ve been exercising a lot and expect and your HRV hasn’t shifted - this just suggests that your body can handle the intensity of your exercise quite well.

Changes in your HRV are not just limited to exercise, but come about by anything that interacts meaningfully with your autonomic nervous system.

Below is a table that represents some of the factors that influence your HRV, but, bear in mind, this list is far from exhaustive.

Context is key when it comes to interpreting your HRV. If your baseline HRV changes, nobody can tell you what that means without context. If you give that data context, like relating it to a recent intense exercise or a recent viral illness, then changes to your baseline begin to make sense.

Now you may be thinking, HRV just tells me things I already know - that I’ve trained hard or that I slept poorly.

If the only purpose of HRV was to reflect what you most likely already know, we would not be interested. The fact of the matter is HRV has the potential, when utilised correctly, to provide tremendous insight.

Utilising your HRV

You do not need to be a pro athlete in order to gain benefits from HRV. The wearable industry has capitalised on HRV through the means of readiness and recovery scores, but these scores just make you aware of your HRV and not how to use it. Below are two examples of how to utilise your HRV:

Exercise Recovery

HRV is a great way to see how you’re adapting to and recovering from exercise. If your baseline HRV takes a dip after a tough training block and you feel exhausted - it may be time to decrease intensity. This does not mean a lower HRV after training is a bad thing. Depending on your exercise of choice, stressing your body is necessary in order to make progress, making HRV an apt metric for coaches and sports scientists when it comes to dealing with athletes and exercise programs.

Detecting Changes in Health

If you are in tune with your HRV and aware of the stressors in your life, a sudden unexplained shift in your HRV baseline may represent an acute illness or flare in a chronic condition. We saw this during the Covid pandemic where unexplained shifts predated symptoms of Covid.

However, HRV is not limited to just diagnosing COVID. The research in this field is at relatively nascent stage, but HRV, in combination with other metrics, has the potential to identify early pregnancy, determine survivability in certain cancers and predict menses. With this potential, and as the use of wearables in healthcare research increases year-on-year, HRV and other biometrics may be fundamental in improving your well-being and building a better you.

The body of research behind HRV is growing and, although further research is needed, the application of HRV in improving mental health and performance through HRV biofeedback, and in predictive modelling to forecast health outcomes makes HRV a heavy hitter when it comes to your wellbeing.

Bottom Line

HRV should be used as a supplement to how you feel, not a replacement for how you feel. Working with your HRV involves knowing your baseline and your life stressors intimately. When it comes to working with your HRV, context is key and A low HRV is not inherently a bad thing and can be extremely informative on how your body responds to certain factors

References

A healthy heart is not a metronome: an integrative review of the heart's anatomy and heart rate variability
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4179748/

A meta-analysis on heart rate variability biofeedback and depressive symptoms https://www.nature.com/articles/s41598-021-86149-7

Heart Rate Variability Biofeedback Improves Emotional and Physical Health and Performance: A Systematic Review and Meta Analysis
https://link.springer.com/article/10.1007/s10484-020-09466-z

“Brain over body”–A study on the willful regulation of autonomic function during cold exposure https://www.sciencedirect.com/science/article/abs/pii/S1053811918300673?via%3Dihub#

Heart rate variability biofeedback: how and why does it work https://www.frontiersin.org/articles/10.3389/fpsyg.2014.00756/full