Heart rate variability and wearables explained

A built-in heart rate sensor is something we take for granted these days when we talk about wearables.

Every smartwatch, fitness tracker and sports device has a built-in heart rate monitor, used for 24/7 monitoring and tracking workouts. But increasingly, heart rate variability (HRV) is the key metric.

It's a heart rate measurement that the likes of Fitbit, Garmin, Apple, Polar Samsung and others use to offer new insights such as stress, recovery, readiness and fatigue. In fact, of the new era of health insights on offer from wearables, HRV is at the heart of them all.

Here's what it means and why it's so important.

• Best heart rate monitors to buy

What is heart rate variability?

Heart rate variability is the measurement of the time interval between heartbeats.

Unlike measuring heart rate, which is about the average number of heart beats per minute, HRV focuses on the small fluctuations of the heartbeat and usually measures that variation time in milliseconds. The higher your HRV, the better.

Those fluctuations can be affected by a whole host of things.

According to Polar, that could be age, body position, the time of day and health status.

Then there's a whole host of mental, physical and emotional experiences that can have an impact on HRV measurements and data. Basically, a lot can impact on your HRV.

The benefits of tracking HRV

Why is it useful to measure or monitor it? Well, for fitness particularly, it's a way of knowing when you're putting too much stress or strain on your body and can be an indicator of whether you're physically and mentally ready to work out again.

So if you're a runner and you've just put in a tough 2-hour run, it'll be able to help you decide whether you're ready to go again the following day or whether you should take the day off from training. When you're working out, your HRV decreases as you heart rate and exercise intensity rises.

But it's not just about serious athletes, as HRV measurements have started to trickle down into devices like fitness trackers where it can also be used to check in on how stressed you are.

What is a normal, high or low HRV?

So what does is mean to have a low or high HRV reading? There's no ideal HRV score, and like resting heart rate, it's hugely personal.

But generally speaking, high HRV as being associated with healthy longevity and the side of the nervous system that promotes relaxation, digestion, sleep and recovery.

A low HRV reading is commonly associated with stress, overtraining, inflammation and illness. Diabetes, coronary heart disease and high cholesterol are things commonly tied to having a low HRV as well.

What is a high, low or normal HRV reading? A blog post on that subject by Oura, makers of the Oura Ring smart ring, which measures HRV during sleep to assess your readiness, offers an explanation:

"“High” and “low” HRV is relative for each person. HRV is a highly sensitive metric, which responds uniquely for everyone.

"Some individuals have steady HRV scores, while others fluctuate greatly. HRV is an evolving tool, which means, at every HRV level, your personal scores and body status observations are especially important."

While Oura has calculated what its research and studies interpret as a normal HRV, it makes clear that every person's HRV is unique and you should look to compare to your own averages as opposed to others to make best sense of the data.

Wearables and HRV:

As we said, most major wearable makers are tapping into HRV measurements for a host of insights.

We've broken down how the key players are using the data to fuel features on your smartwatch or fitness tracker.

Apple Watch

The way Apple treats HRV tracking natively is by continuously recording it with its optical heart rate sensor during your wearing time and letting you view trends inside of the Apple Health app on your paired iPhone. From the Health app, you can see trends over the day, week, month and year to see where there's been increases and decreases in HRV measurements.

If you want delve deeper into what the measurement can tell you, there are third party HRV Apple Watch apps like HRV4Training that can help you interpret the data and offer more actionable data.

Fitbit

Most new Fitbit fitness trackers and smartwatches offer the ability to measure heart rate variability and pay close attention to those variations between heartbeats mainly during sleep.

Fitbit uses a common RMSSD formula to measure HRV, which it now reports directly the Health Metrics section of the Fitbit app. From there, you'll be able to see the latest HRV measurement, which is measured in milliseconds (ms) from the longest sleep period over the past 24 hours.

Basic users of Fitbit can see the HRV trend over the past 7 days, but Premium users can track that for 30 and 90 days.

If you see a significant decrease in HRV, it could be a potential sign of major stress, illness or fatigue. It's worth mentioning that Fitbit this feature isn't intended for medical purposes.

In addition to simply measuring HRV, that metric is used as part of Fitbit's Daily Readiness Scores to guide you on whether you're in good shape to train hard or you should take a day off.

Samsung

On the latest Samsung Galaxy Watch 4, heart rate variability measurements along with heart rate measurements are used to fuel the stress monitoring features available on the smartwatch, which can be done continuously and letting you see on the spot stress measurements directly on the watch.

Garmin

Garmin actually uses HRV measurements across a range of its watches to fuel a host of features and offer a range of insights.

For sleep, it uses HRV to improve the accuracy of determining when you've been awake and the time you've spent in the different sleep stages it's able to recognise. It's also used on newer Garmin watches to generate respiration rate during that sleep time too.

Like Samsung, Garmin also uses that HRV data to track stress levels, which can be done on the spot and continuously with data stored and synced to the Garmin Connect app.

From an exercise and training point of view, it's also a metric that fuels data such as your VO2 max, performance condition, lactate threshold and it powers Garmin's Body Battery energy monitor too.

If you want a better idea of when you should be saving your tough training sessions for, you can also perform an HRV test on compatible watches. You'll need to pair u pan external chest strap monitor to do perform the test and spare three minutes to get a reliable reading.

Garmin doesn't let you see that HRV data in raw, but there are apps you can hunt out on the Connect IQ Store that can help you delve deeper into the data.

Polar

Polar also uses HRV to help you better assess when to train and when to spend time recovering. Watches like the Polar Vantage V2 or Polar Grit X Pro offer an Orthostatic test, which is based on HRV and requires an external heart rate monitor chest strap to conduct the test.

The test results fuel Polar's Recovery Pro features and if you're doing the tests on a regular basis, it can help you better understand whether fatigue is training based or based on another factor.

Oura

We've already spoken a bit about Oura and its smart ring, which is a wearable designed to help you better understand your recovery needs. It tracks HRV overnight in 5-minute samples. it does takes those continual measurements to better inform how you body is responding to stresses and strain you put on the body on that day.

In its companion app, you can then view average HRV, max HRV and HRV Trace and that can be viewed in the Readiness tab and Trends section of the app. You can then use that data to understand how well you've recovered over a day or week and help shape decisions you make about what you do or don't do on a day.

In addition to those, you can also explore HRV Balance, which looks at a longer-term comparison of your current HRV trend and your personal baseline to understand whether you're recovering well or putting your body under serious stress.

How accurate is HRV tracking and what's coming next?

So we've touched on this subject above, but it's worth returning to. To get reliable heart rate variability measurements, the sensors taking the readings need to be up the task. That's when we once again fall into this whole wrist versus chest heart rate debate.

When we spoke to biometrics experts Valencell, they told us, "It's really difficult to deliver heart rate variability from the wrist when you have so much movement of the wrist."

That's likely why you will still see some companies like Garmin and Polar turning to heart rate monitor chest straps to capture heart rate variability and take measurements that require a bit more time than on the spot heart rate measurements, to reliably track that data and generate the most valuable insights into your current state of fitness and stress levels.

Wearable makers have already made great progress putting HRV to use, but in the future it could be also harnessed to tell us more about our health. Apple, Samsung, Fitbit and Huawei have all shown an appetite to delve deeper into serious health monitoring and they may well seek to see what else HRV might be able to tell us about what's going inside our bodies and whether we may be at risk or serious health issues.