Heart Rate Variability (HRV) What Is It And Is It Of Any Use?!
If you’re in the fitness world, it’s very likely that you’ve come across the acronym HRV before. Perhaps the most common place you’ve seen it is on your favourite (or least favourite) fitfluencers instagram stories as they proudly show off their score from their whoop that morning. But what is it, why do they care so much about it and should we or any other population care?! Well that’s what we’re going to dive into today.
Before we dive into what HRV actually is we need to give a little bit of background information on the autonomic nervous system. This is the part of the central nervous system that is responsible for all involuntary actions and it’s made up of the parasympathetic and sympathetic nervous systems.
Central Nervous System And The Vagus Nerve
The sympathetic nervous system is commonly referred to as the fight or flight system. When it dominates, it brings about changes in the body to prepare for action such as:
Increasing availability of glucose
Increasing heart rate, the pupils dilate and blood flow to the muscles is increased with a decrease in blood flow going to the digestive system.
The parasympathetic nervous system is the rest and digest one! When this dominates, changes are made that
Lead to sparing of energy such as a decrease in heart rate.
Increase gut motility and secretion of digestive enzymes.
The vagus nerve is a nerve within that body that plays a huge role in controlling the parasympathetic nervous system! It receives and carries sensory information to and from a lot of organs including the heart. When the vagus nerve is firing, the parasympathetic nervous system is dominant and when the vagus nerve is not, the sympathetic nervous system dominates.
Ok sweet but where does heart rate variability come into all of this?
Well it’s actually a measure of this balance between sympathetic and parasympathetic states. HRV itself is essentially the difference in time between heart beats, measures of HRV will likely use the root mean square of successive differences in heart beats (RMSSD).
Contrary to popular belief the heart is not that regular, your heart might be beating at 60 beats per minute, but it’s not going to be beating every second on the second, there might be 0.8s between one beat and then 1.2s between the other.
This is totally normal and a larger variability is actually indicative of a balance between the sympathetic and parasympathetic nervous systems. If one of these dominates, for example the sympathetic nervous system, then the heart rate will speed up and likely become much more regular with less difference between beats.
If you’re in a state of balance where you’re readily able to switch between parasympathetic and sympathetic then your HRV will be higher.
Think of a gazelle grazing, they might hear a branch snap, the sympathetic nervous system will engage and they’ll be ready for action, once they realise that the noise does not mean danger, they revert to parasympathetic dominance and continue eating and digesting. A low HRV may indicate that they’re less able to switch between systems and might be slower to react to a predator.
Nowadays predators to us are much harder to come by! And instead the stresses of life take a different form, but HRV is no less relevant.
Chronically high HRV scores have been associated with improved well being, mood and productivity while low HRV scores are associated with anxiety, stress and even increased risk of death. BUT it is all about context.
Because our HRV is changing constantly it’s important to look at the time frame of the changes. For example, we can actually control our HRV. When we breathe in, our diaphragm moves down, this actually leads to our heart getting a little bit bigger, this slows the rate of blood flow through the heart and so the heart speeds up! The opposite is also true, upon exhaling our diaphragm moves up, decreasing the size of the heart and thus the heart slows down to control rate of blood flow.
This is a very acute example of what can change HRV. Chronic changes in HRV are essentially linked to stress. High stress environments, changes in training load, being in a calorie deficit and less sleep can all lead to a reduction in HRV over a period of time.
How do we measure it?
Well it’s quite simple really, you just have to be able to measure heart rate. A lot of wearables such as WHOOP or apple watches have the capacity to measure this. They use a technique called photoplethysmography (ppg) that uses light to sense the rate of blood flow through the blood vessels, other methods such as ECGs can also be used. In terms of accuracy, ECGs tend to be a little more accurate than PPG methods but both are still valid.
The key with measurement of HRV is consistency, the best times to measure it are either overnight (which a wearable such as Oura or Whoop may do for you) or first thing in the morning in the “most relaxed” state.
It’s important to take it at the same time each day and there’s absolutely no point comparing your HRV with someone else's, only against your own baseline. Accuracy of wearables or tech always needs to come into the conversation here.
At the minute the most accurate based on research is the use of an ECG, combining a polar chest strap with an app called HRV4 is a relatively easy way to do this. Oura rings are also relatively accurate with a mean percentage error of less than 10%. Interestingly, using a phone app and camera to carry out PPG, the % error is not too far from that of an Oura ring! At this point in time, I cannot find a measure of accuracy for a whoop. This does not mean it won't be accurate! But if it was, maybe they’d want you to know?
So should I monitor it?
I always like to attack a certain question with topics like this, that question being “you can, but should you?”. Due to modern technology, it is relatively easy for us to measure HRV but should you! It’s been found that subjective measures of stress tend to agree with HRV scores, so if you feel stressed and like you need to reduce training volume or increase intake etc then whats the point in measuring it right?
This is a very valid point for those of you that are in tune with your bodies and able to listen to the feedback it gives, but HRV may provide an objective measure that can result in a little more by in.
It’s something that in my practice I use with an Esports team as it’s something that they can visually see and measure and creates buy in with them! It’s also worth using if you’re an athlete with a competition coming up as it will allow you to control training volume a little more to ensure you’re not going into the competition with a low HRV!
Overall it’s not necessarily something that I believe everyone should be using, but there are a tonne of useful applications and when measured and applied in the correct way it’s an awesome tool to have in the toolbox.
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