Heart Rate Variability (HRV) Tracking Devices
We all know about heart rate: the number of times your heart beats in a minute. Lately, a new measure called heart rate variability (HRV) has started to become an essential metric for athletes. The latest Apple Watch measures it, as well as newer devices like the WHOOP Strap, and I predict that more mainstream wearables like Fitbit will soon offer products that do as well.
Put simply, HRV is a measure of the beat-to-beat fluctuations in your heart rate. If you have a heart rate of 60 beats per minute it doesn't mean your heart is beating exactly once per second. Some beats might be 0.9 seconds apart while others might be 1.1 seconds apart.
These variations in heart rate timing are controlled by your autonomic nervous system – the unconscious system that regulates heart rate, digestion, respiratory rate, urination, sexual arousal, and other bodily functions.
Why measure HRV
Trends in HRV can help determine how stressed your nervous system is, whether you're starting to get sick and if you need to rest more after a hard workout day. Therefore, HRV is vital for athletes to determine whether they are properly recovered or not and if they can push themselves harder or need to ease off.
Less variation between heartbeats (a lower HRV score) is an indication that your nervous system is in a sympathetic or "fight-or-flight" mode. More variability (higher HRV) indicates that your body is in a parasympathetic "rest-and-digest" mode.
Neither state is purely good or bad and everybody's HRV is fluctuating throughout the day, but patterns can indicate the general state of your nervous system.
See the image below for an example, which tracks a person's heart rate throughout a day. During stressful times, like public speaking or exercise, you can see heart rate increase (the height of the line) and HRV decrease (the stability or flatness of the line). This makes sense as the nervous system would be in sympathetic "fight-or-flight" mode during each.
At other times of the day, the heart rate is lower and the line becomes more jagged, indicating more fluctuations between heartbeats and a higher HRV.
What does the science say? Studies have shown that low HRV predicts early metabolic disease, heart rate failure, adverse outcomes after a heart attack, and all-cause mortality in the elderly.
In athletes, resting heart rate and sleep need have been associated with changes in HRV.
Popular devices that measure HRV
When measuring HRV it's important to distinguish between devices that measure it without you knowing, such as when you're asleep, and those that measure it while you're watching. HRV is easily manipulated by changes in breathing that can occur unconsciously when you know you're being tested.
- WHOOP Strap – See our review of the WHOOP Strap. This high-end device worn on the wrist is extremely accurate at measuring heart rate and sleep stages. Like the Oura ring, it's unique in that it measures HRV at night so it's a more standardized measure of HRV.
- Oura Ring – This cleverly-designed device is the size and weight of a normal ring and is intended to be worn at night to measure your sleep stages and HRV. I found the ring to be uncomfortable to wear at night, but this device is very popular in the biohacking community. Along with the WHOOP, this device benefits from the fact that is measures HRV when the user is asleep.
- EliteHRV CorSense – This company started with a popular mobile app and has since moved into manufacturing their own device. The CorSense is worn on the finger and looks like a pulse oximeter you might wear at a hospital. The device is unique in that it is sensitive enough to measure high and low-frequency HRV and can be used with a variety of mobile apps.
- HeartMath Inner Balance – This company produces a device that clips onto the ear and is sensitive enough to measure high and low-frequency HRV. It comes bundled with its own mobile app.
- Apple Watch – With watchOS4 Apple introduced HRV to their watches. There have been doubts about the sensitivity of the Apple Watch, but at least one study has indicated that the device is a reliable source of HRV.