Why Watches and Wrist HR Monitors Usually Don't Work For HRV
Elite HRV works with any sensor or device that transmits accurate R-R intervals via Bluetooth 4.0+ protocol.
Different devices are designed for different purposes. Most watches, arm bands, and fitness heart rate trackers are designed for heart rate only. This means that they do not accurately measure R-R intervals. R-R intervals are the exact time measurement in milliseconds between each heartbeat that are needed for truly accurate Heart Rate Variability (HRV) calculations. These millisecond times between each beat are also called IBIs or inter-beat intervals.
Due to the limitations of those hardware devices, we generally do not recommend them for reliable HRV readings. These devices are fine for just getting your basic heart rate though! Many devices claim to be "ECG accurate", but they are not specifying for what parameters. Most of the time accuracy claims are referring to heart rate (beats per minute, BPM) or a non-HRV measure, such as detection of atrial fibrillation, which has different requirements than HRV calculations.
Examples of devices that do not measure HRV include: the Fitbit Charge HR, Mio wrist devices, Polar wrist watches, Apple Watch, etc. We hope the hardware will advance sometime soon and provide accurate R-R intervals (We know they are very convenient!).
There is no affordable hardware or sensor on the market that can measure accurate HRV from the wrist, arm, or hand using optical sensors or PPG while you are moving. You must remain still (within reason) to accurately measure HRV using that technology in those locations on the body.
In the meantime, the best option is to invest in a chest strap heart rate monitor or an HRV-specific device like the CorSense. These devices are built for accuracy first and specifically for Heart Rate Variability.
Some devices with a "closed system" claim to measure HRV, but access to the data is not allowed, so we cannot comment on the reliability of these systems. In order to calculate HRV values, we must receive the R-R interval data or full access to the raw sensor data.
For the Garmin Vivosmart 3 specifically, Garmin Customer Service reports that the sensor is not intended for HRV analysis by users or by 3rd party apps (more info provided by Garmin here).
For a list of heart rate monitors that are accurate enough for Heart Rate Variability measurements and give you full control over your data, see this page: https://elitehrv.com/compatible-devices
More about R-R intervals or inter-beat intervals (IBIs): What are RR intervals?