vivosmart 4 fitness tracking

This week, Garmin unveiled a new feature in their Vivosmart 4 fitness tracker intended to set the device apart: a “body battery” that can estimate energy levels and provide insight into how stress, workouts, and sleep are affecting them. It doesn’t actually present anything new on the hardware side, but the updated software behind the body battery has a few useful updates that are worth taking a closer look at.

The innovation behind the body battery comes from the analytics team at Firstbeat Technologies Oy, a Finland-based company that provides analysis programs that allow 32 different Garmin sensors to make sense of all those heartbeats. For anyone familiar with Garmin’s triathlon or running watches, Firstbeat is the company responsible for Garmin’s existing “recovery advisor” (which estimates how many hours you’ll need to feel recovered after a workout) and All-day Stress tracking analysis.

The technology and heart rate science behind the All-day Stress tracker and the new Body Battery are nearly identical, Firstbeat representative Herman Bonner tells Inverse, but the version of their technology in the Vivosmart 4 has been updated to integrate sleep data. This, he says, will help users distinguish between stress from workouts and stress from daily life. It also incorporates the battery visual so users can now track restorative moments, not just stressful ones.

garmin fitness tracker
Garmin's Vivosmart 4 tracker, which features a body battery 

“The Body Battery is new, but All-day Stress tracking, which is closely related, has been available in number of Garmin devices for the past year,” he says. “The feedback you get from Body Battery is based primarily on an analysis of heart rate variability (HRV) data, the same analysis used to deliver Garmin’s popular All-day Stress tracking.”

Tweaking the HRV

Garmin’s stress tracking technology is based on the concept of “heart rate variability” (HRV), Bonner says.

If you look at a chart measuring heartbeats from an EKG, you’ll see the familiar pattern of peaks and valleys. But products that analyze Heart Rate Variability look at the time interval between the highest peaks between two consecutive heartbeats, also called an “R-R” interval. Several papers, and one meta-analysis literature review of 37 papers found that varability in the time between two consecutive heartbeats is a good proxy for estimating the activation of the autonomic nervous system — in short the branch of the central nervous system that activates the “fight or flight” response to stress.

HRV
Firstbeat's illustration of HRV in the R-R interval

Strangely, slightly irregular R-R intervals are a good thing. The more regular they become the more it may indicate that the body isn’t recovering well from previous stressors — this might drain the body battery. But during some kind of relaxing activity, HRV may become irregular. That might help refill the battery.

“Among the more memorable things we’ve seen in our Lifestyle Assessment data are people recognizing that a phone call from their spouse or time playing with their kids actually provided a boost to their energy levels,” Bonner adds.

Firstbeat isn’t the only company to see the potential in analyzing HRV. It drives other “stress trackers” on the market and is used by companies like Cardiogram to track heart conditions, and research suggests it may also be useful for determining stress levels.

Workout Mode versus All-Day Tracking

This function will make it possible for users to figure out what drains the body battery faster: A tough workout, which might cause short-term exhaustion, or a day of inactivity filled with constant stressors? The body battery appears to be able to distinguish between the two, says Bonner.

“A tough workout will drain your Body Battery at a faster rate, but excessive stress sustained over the course of a work day might have a large cumulative impact,” he explains. In workout mode, he says, Firstbeat determines impact on the body slightly differently by focusing on duration and patterns of intensity during workout that allow it to estimate how long it will take the body to recover.

Add to that the idea that the body battery can move in both directions, and it seems that Garmin is taking a step into the lifestyle-tracking space.