What is Heart Rate Variability?

Heart rate variability, (or HRV) is yet another acronym to add to our vocabulary and a data point that we can (and should?) track to help build a picture of our fitness.

With the continuous development of intelligent wearable tech emerging on the market, we suddenly have all of this information at our fingertips, something that can be quite overwhelming. But how much of this information do we need to use, what provides the most helpful data points and is HRV something we should track?

What is Heart Rate Variability?

When you think of a heartbeat, do you think of a steady, rhythmic bu-boom… bu-boom… bu-boom? To the human ear or touch you’d be correct. However, rather than a metronome ticking consistently, there are actually slight fluctuations between heartbeats, detectable only by specialised devices. 

You might also be surprised to know that these fluctuations in your heartbeat can be a very good thing and an indicator that your body is ready to execute at a high level. However, it’s a little more complicated than just aiming for a high HRV. 

The variation is controlled by the same system that works behind the scenes to regulate and control our regular bodily functions such as breathing, digestion and blood pressure and reacts to many different stimuli from a poor night of sleep or an unhealthy diet to a piece of fantastic news or seeing a loved one. This system is called the autonomic nervous system (ANS) and can be split into two branches:

Sympathetic nervous system: referred to as your “fight or flight” response, causing an increased heart rate.

Parasympathetic nervous system: a state of “rest and digest”, causing a decreased heart rate.

Your heart rate variability comes from these two systems competing simultaneously to send signals to your heart, each one aiming to increase or decrease heart rate, thus resulting in the fluctuations we track with HRV.

What affects HRV?

Given that HRV aligns with your autonomic nervous system, when you are in a more relaxed state HRV increases. And vice versa: in states of stress, HRV will decrease. Put simply:

Low HRV: A low variability in heart rate means you are working your sympathetic nervous system and are in a state of fight or flight.

High HRV: More variation means the parasympathetic nervous system is activated and you are in a more relaxed state, associated with good recovery.

There are other factors such as age, gender and genetics that will also play a role in your HRV.

Should you aim for a low or high HRV?

Based on this, we should aim for a higher HRV, right? 

Well, it’s a little more nuanced than just aiming for low or high.

There are some general figures we can look at and typically, values below 50 ms are classified as unhealthy, 50–100 ms signal compromised health, and above 100 ms are healthy.

While a higher HRV indicates better health and fitness, there are instances where a higher HRV can indicate pathological conditions such as cardiac conduction abnormalities.

Also, a lower HRV isn’t always something to be concerned with; it is not uncommon for a particularly intense training session or bad sleep to decrease the following day’s HRV. With proper recovery, this will move closer to normal in a few days.

So, how can we use HRV?

Your HRV is highly sensitive and specific to you, and therefore, asking “What is a good HRV?” is not the most helpful way to use the data. The true value lies not in comparing your personal HRV with general population norms, but rather in examining the trends that emerge over time within your own HRV data.

Whoop, one of the first wearable trackers to use HRV as a prominent metric says that HRV is “widely considered one of the best objective metrics for physical fitness and determining your body’s readiness to perform.”

If you are on a journey towards better health, you would likely see a gradual increase in your average HRV as your fitness and resilience to stress increase.

By examining your HRV on a daily basis, you can uncover meaningful trends about how different stimuli affect your body. This can be anything from training, sleep patterns, and dietary choices to emotional states, thoughts and feelings. By analyzing these connections, you gain insight into which actions positively or negatively impact your HRV, helping you to make more informed decisions that enhance your day-to-day life.

For those who have a fitness goal, HRV serves as a valuable gauge for determining when to push and when to pull back with training. A higher HRV shows that your body is in a recovered state, ready to take on a harder session. By leveraging this insight, you can work smarter, not harder, and save the more demanding workouts for the days when your body is sufficiently recovered. This strategic approach enhances overall efficiency and performance, helping you to stay on track and move fast towards your goals.

If you are noticing a downward trend over a period of time, this could indicate that there are chronic stressors keeping your body in a fight or flight mode. This could be from a range of factors such as you are not rested and recovered, perhaps from overtraining, illness, stress, lack of sleep or other similar indications.

I want to see my HRV trends, how do I track?

Without diving too deep into the science behind tracking HRV, (although if you’re interested, Frontiers do a great explanation of the data and metrics collected) the best way to measure HRV is to look at a long strip of an electrocardiogram (ECG), taken by a doctor.

Most of the widely available wearable tech, however, use photoplethysmography (PPG) which uses the blood flow under the skin to calculate the inter-beat interval (IBI) or R-R Interval (RR). 

Whoop calculates HRV using RMSSD, the root mean square of successive differences between heartbeats. The tech will then take this data and turn it into an easy-to-understand metric, in their case the “Strain” score, which on a day-to-day basis will give you an assessment of your body’s state of readiness. This, coupled with a journaling feature that allows you to track what you get up to can provide a quick and clear picture for you to use.

Uses wear their Whoop strap to bed which enables it to track to use your last slow wave sleep stage each night to determine your baseline to monitor and track against. Each morning, it uses your HRV (as well as your resting heart rate, respiratory rate and sleep performance) to calculate your daily recovery–how ready your body is to perform.

Other trackers such as the Oura ring, which provides a ‘Readiness Score’ and Garmin watches which has an HRV Status Widget and a ‘Morning Report’

Apple Watches also have the ability to track HRV data but unlike Whoop, it does not convert it into easy-to-digest data in the same way Whoop does. It would be down to you to look at the trends and figure out the meaning for yourself.

How to improve your HRV

There’s not one quick fix to improve your HRV trends and many of the ways to improve it are general health-benefitting actions – hydration, quality sleep, maintaining a routine aligned with your circadian rhythm, avoiding alcohol, eating well and prioritising recovery.

The takeaway

HRV is a great, easy-to-access (for a slight fee) piece of data that is particularly useful for those who have goals in the gym and want to lead active and healthy lifestyles. 

However, it is not something that is vital for everyone to track. If you are in tune with your body, take the time to assess what works for you and what doesn’t.

Helpful, yes. Essential, no.

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