An introduction to heart rate variability (HRV)
Continuing the Tri Training Harder theme of recovery for this week, coach Jon Reilly takes a look into heart rate variability (HRV) as a tool for measuring recovery. What is it, does it work and could it be used as an early warning for COVID-19?
What is HRV?
Historically, there have been several methods for assessing levels of recovery or fatigue, such as training diaries, training stress (TSS/ATL in Training Peaks) or resting heart rate. However, more recently HRV has become widely used by athletes around the world. It has become recognised as a more accurate measure of the effects of stress on the body.
HRV is used to describe the small differences in time intervals between heartbeats. If your heart rate is currently 60 beats per minute, it is not beating exactly once every second. Within that minute, there may be 0.9 seconds between two beats and 1.1 seconds between the next two beats. The greater this variability, the higher your HRV.
What can HRV tell us?
HRV is controlled by your autonomic nervous system (ANS). The ANS controls the involuntary aspects of your physiology via two branches - parasympathetic (‘rest and digest’) and sympathetic (‘fight or flight’). HRV comes from these two competing branches simultaneously sending signals to your heart. If your nervous system is balanced, your heart is constantly being told to beat slower by your parasympathetic system and beat faster by your sympathetic system. This causes fluctuations in your heart rate and gives a high HRV reading. This is a sign that your nervous system is balanced and that your body is ready to perform at its best.
If you have low heart rate variability, one ANS branch is dominating and sending stronger signals to your heart than the other, this is usually the sympathetic branch. There are times when this is a good thing. If you are exercising you need your body to focus on allocating resources to your muscles (sympathetic) rather than your digestive system (parasympathetic). However, if you are at rest, low HRV indicates your body is working hard for other reasons. It could be a temporary response to a previous day’s hard workout or a poor night's sleep. According to ithlete, a small drop in HRV is okay, as long as you prioritise recovery. After all, fitness improvements come about by applying stress and allowing recovery. A hard session, especially on top of accumulated fatigue, is likely to lower your HRV. If your HRV stays low even with rest you could be fatigued or about to become unwell.
Factors affecting HRV
Many things can have an impact upon your HRV, according to Whoop.com, the following factors can reduce your HRV:
Over-training - Pushing too hard for too many days without allowing your body to recover can cause your HRV to drop.
Dehydration - Dehydration can reduce the volume of blood plasma, which means your heart has to work harder to circulate and deliver oxygen and nutrients to your body, this extra load can decrease HRV.
Alcohol - Data has been able to show a relationship between decreased HRV and alcohol consumption.
Poor diet - Poor nutrition and eating at abnormal times of day can have adverse effects on HRV.
Poor sleep - Inconsistent, poor quality sleep has been shown to decrease HRV values.
How to track HRV
HRV tracking apps, like the ones listed, use your phone’s camera or a heart rate strap (which is more accurate), to record your daily HRV score:
There are lots of apps available, for a review of a number of HRV apps visit this article.
As well as apps, many smart and multi-sport watches now incorporate HRV tracking. One such wearable is the Whoop strap which combines HRV, resting heart rate, respiratory rate, sleep, and strain data to give you a daily readiness score. This helps you to assess your state of recovery and plan your training for the day.
Does it work? - My take
I’ve been tracking my HRV every morning for the last few months using Elite HRV. When comparing my HRV scores to my fatigue levels (ATL on Training Peaks) my HRV seems to be high when my ATL is high, which is opposite to what I would expect if I were overdoing it, so in my case my HRV looks to be following my fitness level.
I have certainly noticed that when I’ve slept badly or drank a little too much alcohol this has been reflected in my HRV reading the next day. The main thing I have learnt from my experimentation with HRV is that consistency in recording is key. This Training Peaks article offers some useful tips on collating reliable HRV data.
Can HRV warn us about COVID-19?
In short, no.
According to Elite HRV, abrupt drops in HRV and increases in resting heart rate can indicate an impending illness. However, both metrics are affected by many other variables such as training load, stress, alcohol consumption, and quality of sleep therefore just because there are changes in HRV it doesn’t necessarily mean you will get ill.
Whoop have been analysing their users’ data and have surmised that rather than looking at HRV, resting respiratory rate could be a better indicator for possible early COVID-19 detection. Their data suggests resting respiratory rate is normally very stable, a sudden increase in respiratory rate over a few days could indicate a possible impending lower respiratory tract infection, such as COVID-19. Read Whoop’s latest blog to find out more.
In summary, consistently tracking HRV can be a great way of monitoring your recovery. It can indicate when to lay off the training and when to push on. It may also give you an early warning for impending general sickness. In this period, while we are training less, focussing on our bodies, strength and recovery, now could be a great time to introduce HRV monitoring into your daily routine. Discuss HRV with your Tri Training Harder coach to find out whether it could work for you. If you are already tracking your HRV, now is the time to focus on recovery, even if your app says you are primed to perform.
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