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When it comes to Fitbit features, heart rate variability (HRV) is among the most controversial. It represents an effort by Fitbit to play catch up when it comes to some of the more advanced features being offered by many companies in their smart fitness tracking devices.
The controversy enters the picture when it comes to the methodology and performance of Fitbit’s HRV feature. A look at any forum thread regarding Fitbit and Fitbit performance will reveal an eye-opening number of posts about how wildly inaccurate the company’s HRV feature can be, so it definitely warrants a deeper examination.
How HRV Works
Heart rate variability refers to the fact that while our heartbeats are regular and predictable, they’re hardly metronomic. The time between beats can vary from between 20-200 milliseconds in a normal heart, and while that might not seem like much, it can make a huge difference when it comes to workout performance.
HRV does vary among individuals, however. The factors that cause this variation include the obvious ones-age, gender, genetics, general fitness level-and some less obvious ones like hormone levels, diet and nutrition, lifestyle choices related to stress, caffeine, and alcohol use, and so on.
Because of this, individual comparisons can be misleading. As a general rule, the higher the number the better the fitness level, as a higher HRV is usually associated with higher responses from the parasympathetic nervous system, which governs things like sleep and recovery.
A lower HRV number, meanwhile, indicates that the autonomic nervous system is taking control, with responses associated with fight-or-flight reactions, stress, overtraining, and illness.
But it’s important not to take the numbers too seriously. These aren’t lab-based measurements, and they’re basically intended to be used as information sources only.
How Fitbit Calculates HRV
The most interesting thing about the way Fitbit tracks HRV is that it’s monitored primarily in sleep, with no way to track it during the day. Also, the tracing feature only comes on when you sleep for at least three hours.
In theory that means it tracks recovery better than some devices that do 24-hour tracking, in part because it eliminates the variability that comes with issues like caffeine use, what time of day you work out, and so on.
The idea is to come up with a baseline set of numbers for you to evaluate your recovery, so you can adjust your workouts accordingly.
Is Fitbit HRV Accurate?
The accuracy issue is where controversy enters the picture. Earlier Fitbit devices relied on an optical sensor to calculate HRV, and this may have been responsible for the large number of accuracy issues that were reported in various fitness threads.
To counter this issue, the Fitbit Charge 5 now uses an electrocardiogram that’s been cleared by the FDA and can be used to detect arrhythmia.
But several studies have pointed out that inaccuracy continues to be an issue with HRV measurements. Some of the more expensive, state-of-the-art devices are more accurate, of course, but in general, it’s a good idea to use the numbers you get as a baseline only, and not to read too much into a high or a low number.
Fitbit Devices and Tracking Procedures
Fitbit’s HRV information is available on a limited number of devices. These include the Charge 2+, the Ionic, the Luxe, the Versa Series, Sense devices, and the Inspire HR+.
It’s important to check the model number as well, as you may need a subscription to Fitbit Premium to be able to access the health metrics.
To make sure the data is as accurate as possible, it’s also important to make sure you wear the device for a full day, and this includes your sleep time. You need to get a solid fit, too, and some users have reported getting better readings by wearing the Fitbit a few inches above the wrist bone.
Fortunately, there’s less variability in using the Fitbit app to access data. To get to the numbers, tap Today, then choose Health Metrics and scroll down to the Heart rate variability (HRV) section.
You’ll see a daily graph of your HRV rate expressed in milliseconds, and over time a clear pattern should start to emerge that you can correlate with your workout performance.
Once you have this baseline, it’s important to use it as an indicator only. Fitness threads are full of posts from Fitbit users who are wildly confused by Fitbit readings that are basically impossible to interpret, and some users are getting no readings at all.
How to Use Fitbit HRV Information
The Fitbit HRV is supposed to give you a mirror readout that gives you an idea of how your autonomic nervous system is functioning, but it’s definitely not a medical device, nor should it be used as such.
The best way to use the HRV data is to correlate it with your workout patterns. The data will be more useful if you’re doing regular workout patterns, as this will give you a potentially solid way to measure recovery.
It may also give you some nutritional clues related to your workouts as well. Poor nutrition can reduce your HRV, and insufficient hydration can also contribute to a lower number.
Finally, there’s the importance of good sleep hygiene. Using exercise and relaxation techniques to reduce stress can lead to better sleep, and this in turn can raise your HRV score.
What you eat and when you eat plays an important role, as well as monitoring your screen time at night to make sure you get sound, restful sleep.