A built-in heart rate sensor is something we take for granted these days when we talk about wearables. Whether's that's a fitness tracker that records bpm readings 24/7 or a GPS running watch that wants to help you out with a session of high intensity interval training.
We know a lot of those optical sensors are not great at the moment, but swapping long runs for shorter bursts of heart rate zone training and understanding resting heart rate can definitely change the way you think about keeping fit and healthy.
Read next: Best wearables for HIIT workouts
Like many, we'd imagine, it's another layer of fitness data that you really need to get to grips with before it can start having an impact. Counting steps and distance is pretty straightforward, but things begin to get more complicated when you start throwing heart rate into the mix.
The latest piece of biometric data that everyone seems to be talking about is heart rate variability. Something that has been introduced to the Fitbit Charge 2, and most recently the Garmin Vivosmart 3. It's also beginning to crop up in smart clothing and other wearable devices as well.
We've hit the books (did some Googling), spoke to some experts and now have a much better idea of why HRV tracking is important and why we hope it will make it into more wearables in the future.
What exactly is heart rate variability?
So, here's a breakdown of heart rate variability. Every heart rate varies and so with HRV, it relates to 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 heart. Now 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. A whole host of mental, physical and emotional experiences can impact on HRV.
The benefits of reading 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.
Read this: Stress beating tech to keep you sane
So if you're a runner and you've just put in a 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.
LifeTrak, makers of the Zoom HRV wearable has a very good break down of what it means to have a high or low HRV reading. Essentially, a high HRV is strongly 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 and inflammation. Diabetes, coronary heart disease and high cholesterol are things commonly tied to having a low HRV as well.
Getting a measurement
This will of course vary by device, the tech it uses to monitor heart rate activity and the data it wants to present once it has been read. So in the case of the Charge 2 and the Vivosmart 3, these two trackers are concerned about finding out how stressed you are. So they rely on the optical heart rate monitors to take these heart rate variability measurements. That HRV data is not displayed in the respective companion apps, instead analysing and interpreting those beat-to-beat changes from the wrist in real-time providing a stress score in the case of the Garmin fitness tracker or suggest a personalised breathing program, which you'll get with the Charge 2.
Sports watches from the likes of Polar, Garmin and TomTom also use the data to provide insights into recovery between workout sessions. These watches typically rely on heart rate monitor chest straps to do the job, but as optical sensors continue to improve, like the one on the Spark 3 for instance, that data can be taken more reliably from the wrist as well.
In the case of the LifeTrak Zoom HRV and the Jaybird Reign, one of the first wearables to offer HRV readings, you're able to take on demand readings that usually take a few minutes before delivering a HRV score. These readings are generally best taken in the morning and there's a very good reason for that. That's because it minimises the variables (stresses) that could impact on the data. That could be running around trying to sort the kids out for school or sitting in traffic for hours.
The whole accuracy thing
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 recently, this is what they told us on the subject:
"It's really difficult to deliver heart rate variability from the wrist when you have so much movement of the wrist. Our technology works best when people are not moving. We are developing some technology that we are launching later this year that takes care of the wrist situation for this lifestyle motion issue. The ear is much more straightforward. If you have an earpiece on, we can monitor that HRV data that can be used for stress analysis, arrhythmia detection and much more."
This is why the likes of LifeTrak offer an alternative method to taking readings from the wrist, to improve accuracy for the sake of waiting a few minutes to find out. It's also why smart garments from the likes of Hexoskin and startup Vitali, who embed the heart rate sensors into the clothing so they are in close proximity to the heart are considered more reliable for HRV measurements.
Bottom line, it's an exciting time that more wearables are being able to tap into heart rate variability whether it's for improving fitness or just being able to know when you're feeling a bit stressed. This is really just the start of the relationship between the two, so don't be surprised if more HRV tracking devices are on their way over in the not too distant future.