Heart rate tracking on earbuds was probably a mistake

Athletes typically choose a heart rate chest strap, armband, or fitness watch to judge health and effort. However, a relatively niche (and strange) option is wearing wireless earbuds with heart rate monitoring. I only recently heard of this tech and immediately decided to test whether earbud heart rate accuracy is good enough for serious athletes.

When I read about the Sennheiser Momentum Sport, which sends HR data to the Polar Flow app, I wondered whether Polar H10 users would want or trust an earbud solution or if this targets more casual fans. 

Some athletes like standalone LTE smartwatches to work out phone-less, but I don’t know of any athlete who’d want to go watch-less for workouts, especially since you’d lose things like GPS, running form data, and a quick-glance screen. So rather than replace a fitness watch, perhaps these fitness earbuds could provide more accurate data.

I decided to test multiple wireless earbuds with heart rate, but the more I looked, the more I found one-off, discontinued products like Bose SoundSport Pulse and Jabra Elite Sport. It made me wonder why these brands didn’t bring HR tech to other workout earbuds: consumer apathy or unreliable results?

I tested the Sennheiser Momentum Sport and Anker Soundcore Liberty 4 earbuds against my favorite Garmin fitness watch and a COROS Heart Rate Monitor across multiple runs and walks, to judge whether or not I can rely on earbuds for accurate workout data.

Earbud heart rate accuracy test

The Anker Soundcore Liberty 4 only makes its workouts available through the companion app’s “Wellness” page. You can’t export your workouts to other fitness apps like Strava or even see your workouts unless the earbuds are connected. The barebones workout results page shows an HR average, chart, and set HR zones that I couldn’t edit.

In other words, Anker’s decision to force you to use its app makes the data all but useless to athletes. Still, I’d already run three miles wearing the Liberty 4, Garmin Forerunner 965, and COROS HRM synced to another COROS watch, so I did my best to interpret the data. 

The COROS HRM determined a 147bpm average across my three-mile run. My Garmin Forerunner 965 measured 148bpm; the chart above shows moments where it dips more slowly than the armband when my pace slows or misses small changes in heart rate. It generally matches with few errors—what you’d expect from a reliable optical HR monitor.

As for the earbuds, Anker calculated a 149 bpm average, which doesn’t sound too imprecise at first glance. But looking at the heart rate graph closely, it has me hitting 167 or 168bpm for sustained stretches during my run; neither Garmin nor COROS measured me any higher than 159bpm. 

There’s also the simple fact that as I ran with the Soundcore app open, my heart rate seemed to fluctuate much more erratically than my watch or HR strap. It may average out to something close to the right result, but it didn’t leave me feeling confident. And in the end, the right earbud — which tracks heart rate — burned through its battery much more quickly than the left.

I switched to the Sennheiser Momentum Sport since it exports to the Polar Flow app; this meant I could actually compare heart rate charts directly and better test earbud HR accuracy. To start, I did an impromptu four-mile walk with my earbuds and Garmin watch, followed by a five-mile run.

For the walk, Sennheiser fell short of Garmin by 1 bpm; since Garmin often averages about 1 bpm above my arm and chest straps, that’s not a bad result. I did notice a few anomalies in the graph where Sennheiser had my heart rate leaping well above my actual effort level, but generally speaking, it followed my Forerunner data pretty closely.

My run results, for comparison, were disappointing:

Things started out swimmingly. Sennheiser once again had a couple of elevated blips in the early miles but settled down and tracked Garmin closely through the first 3.5 miles. Then I decided to pick up my pace, at which point the Momentum Sport couldn’t keep up: they averaged about 10bpm lower, sometimes only 5bpm down but frequently up to 15bpm off.

Sennheiser finished with a 153bpm average, only two short of Garmin’s estimate. That’s perfectly adequate for an everyday athlete looking for data on burned calories, but it’s not accurate enough to make a serious athlete happy.

I decided to do one more proper test. I used Sennheiser’s fit test to confirm I had the correct ear cup size, put on my COROS strap and Garmin watch, and did one more run split into two activities: one at a slow pace, one a hard track workout.

Imagine my intense frustration to discover that Sennheiser and Polar failed to sync properly. For both my short run and track workout, the Polar Flow app stopped receiving new information and locked my heart rate at a set point. I didn’t notice because I kept my phone in my pocket during runs. 

You might assume that you have to keep the Polar Flow app open, but that wasn’t the case with my previous tests. Plus, you can see the track workout above showing that it suddenly started receiving heart rate data near the end, with my phone still in my pocket. For that brief glimpse, it was more of the same: the Sennheiser Momentum Sport was about 10–20 bpm short of my actual heart rate. 

Niche for a reason

I began this earbud heart rate accuracy test totally optimistic about how the results would turn out. I had a vague sense that my ears would provide a dependable window to my heart, the same way you can check someone’s pulse with either their wrist or neck. Instead, I came away disappointed.

I never got any results that suggested I should trust workout earbuds as much as a fitness watch; the wonky data reminded me more of smart rings, which are accurate in ideal conditions but can struggle with accurate workout data and higher heart rates. Plus, I missed wearing open-ear earbuds, which feel much safer and more comfortable during runs.

The Sennheiser Momentum Sport also tracks body temperature during workouts, but that was also a letdown.

I used a pro-level temperature sensor during a track workout in March and discovered my body reached a low-grade fever (101ºF / 38.3ºC) at max effort. Sennheiser said my body heat never rose above 97.8ºF. I suspect outside airflow kept my ear canals cooler than the rest of my body and that these earbuds would work better for an indoor gym workout. 

A weightlifter who hates wearing watches at the gym could get away with wearing heart rate earbuds instead and get decent results for a general caloric burn number. Otherwise, I just don’t see the need for this technology, at least until one company can achieve more consistent data.