Optical heart rate monitors on the back of watches are known to be unreliable. I wanted to see how the optical, strap sensor from Coros performed when compared to a chest strap heart rate monitor.
I’ve recorded my heart rate whilst training for well over 15 years. I’ve experimented with wrist based sensors but found them to be extremely unreliable compared to a chest strap so for the vast majority of my runs I have used a chest strap. I’m currently using the Garmin Run strap paired with a Garmin Fenix 5 watch.
I’d heard that optical sensors worn on your arm tended to be more accurate than wrist sensors so I thought I’d check out the claims. The Coros heart rate monitor strap still uses an optical sensor but it is designed to be worn on the forearm or upper arm.
Coros optical HRM strap
The Test
In this experiment I went for a run whilst wearing two watches, each one recording heart rate independently of the other. On my right wrist I wore the Fenix 5, paired with the Garmin chest strap and on my left wrist I wore the Coros Apex 2 Pro.
2 watch Taylor!
The run was done at an easy effort on undulating trail terrain. For the first 15 minutes I recorded heart rate from the wrist sensor on the Coros, then I switched over to the Coros arm strap which I wore on my upper arm. I then continued for a further 15 minutes at the same pace on similar terrain. I then took the arm strap off to see if the Coros watch would revert back to its wrist based sensor. The results were interesting!
Coros vs Garmin heart rate monitors
I wore the strap on my left upper arm
The results
I uploaded the activities to the relevant platforms, namely Garmin Connect and the Coros Training Hub. From there I exported the data as .fit files (this saves heart rate data etc). To analyse the data I imported the files into a free site called EnDuRA (https://bluecattechnical.shinyapps.io/EnDuRA) which I recommend if you are into geeky stats!
The graph below shows the results with the Coros trace in grey and the Garmin trace in darker blue. The first 15 minutes (900 seconds) show a large discrepancy between the Coros wrist sensor and the Garmin chest strap, with the Coros reading higher. Note at 306 seconds the Coros watch read 160bpm whilst the Garmin only read 133bpm. As it was a slow, easy effort I can guarantee that my heart rate wasn’t actually 160!
The gap in the middle of the chart is where I paused both watches to put the arm strap on and you can see that when I restarted the watches and resumed running the traces were much more aligned, at some points giving identical readings. Just before 2000 seconds I paused the watches again and took off the arm strap. When I restarted the watches the Coros watch failed to revert back to its wrist based sensor. So for the final segment of the run, other than a couple of erroneous blips, only the Garmin hear rate is shown on the chart.
Comparison chart
Conclusion
For people who want to record an accurate heart rate during exercise but don’t like wearing a chest strap, the optical arm strap looks like a good option. This was the first test done at an easy pace. It clearly shows that the Coros arm based optical heart rate monitor gives a similar reading to the Garmin chest strap. The wrist based sensor reads much higher than the both the arm sensor and chest strap, and my perceived exertion!
I intend to do more tests at higher intensity and whilst doing interval training. Watch this space for more results!
See video below where I discuss the results.
