I trained a lot with a chest HR for years and I know what 100bpm, 140bpm or 170bpm feel like. When doing trail runs in mountains I have very steep uphills. I have to walk with poles. I'm talking 15% grades over technical terrain. My flat running pace would be 5:00/km, in this kind of very steep uphill it can drop to 12:00/km. My heart usually reaches 160-170bpm, i can hear my heart pumping, I sweat, I can't talk. Now since i have my garmin 5x with optical HR, my reading on my HR drops dramaticly to like 100bpm on these ups. It's impossible. At first I though it was the jerking of running, but I found out it is fairly accurate when running downhill and flats when much more jerking occurs. Its also accurate cycling.
It must be some kind of algorithm to eliminate outliers, like mistakes of the optical HR. This "clipping" algorithm kicks in when the pace drops to walking speed (or even slower) because it must assume "it's impossible someone running doing 12:00/km to have a heart rate of 170bpm). I think this is poorly though for a device done for trail running and climbing, not just road running.
I have two examples if you want to see.
This with a chest strap, notice the bumps on the HR when I climb:
https://www.strava.com/activities/380044957
And this is with the optical HR garmin 5x. Notice the drop on HR on every uphill and how it correlates to plunges in pace.
https://www.strava.com/activities/1118717277
There should be an option to turn off this kind of control or whatever this feature is, specially for sports like trail running.
