Forerunner 935 erratic heart rate

7157910 said:

FR 935 heart rate suddenly jumped to 180 and stayed above 170 for about 2 min

It probably locked in on your cadens. Mine does that all the time. Especially when it's cold outside. 

Last month I ran a 10k competition on a track. Until halfway my heart rate was 160+, then it suddenly dropped to 130 and stayed there till the finish. I can assure you, my heart rate was 160+ because I was going flat out. Can't explain what happened.

The optical heart rate sensor is at best only good for resting heart rate (not only my current FR 935, but also my previous Tomtom). For all others I use a chest strap (or accept that I have no correct heart rate data, when I do a 10k in competition, and don't want to wear a chest strap)

Thank Elbo. Yes, it was cold in lower 40th.

Hi, I have similar issues with my FR935. The heart rate is incredibly erratic for the first 10 mins or so of any run, after which it settles down to more or less accurate (i.e agreeing with measuring manually).

I get the same using the optical HR or with a chest strap.

Germin support recommended warming up for at least 2 mins first, to get my HR to running pace, because of the algorithm that it uses, but I tried that today and it still spiked to 160 BPM when in reality I wasn't going over 120 in the first 10 mins. When I stopped, it went back to the correct value, and as soon as I started running again, it spiked to 25-50% over the correct value. Walking also caused it to spike.

This is all using a chest strap, BTW, so shouldn't be affected by cadance or wrist strap tightness etc.

One idea that I've just had is to see what the HR on my bike computer says when I'm running (I have an Edge 1000) so I'll give that a go and let you know how the 2 devices compare using the same chest strap. I also have a spare chest strap, so I might try comparing results between these too...

Thanks, Ian. Recently it is very cold in my place, in upper 30s or lower 40s when I run. I always wear both FR935+H10 and Vivosport oHR. What I notice is that near the end of my typical 5-6 mile run, Vivosport jumps up to high 150 while FR935+H10 stays around mid 140. I always wet my H10 before running. Given the 10+ difference, I am a little bit concerned. Btw, I always use ANT+ for FR935+H10 pairing. I will try BLE to see if it is the connection issue, which I doubt. 

I've done a little research.

First of all, I went for a run tracked by my FR 935 and my Edge 1000. Both were connected to the same HR chest strap. Both gave exactly the same HR results and both were wildly wrong, reading 20-50% over during the first mile / 15 mins. I ran deliberately slow, close to walking pace, and my HR was reading 160 on the FR and EDGE, but only 90 when checked manually. Slowing down to walking reduced it a bit, but I had to stop for the Garmins to get to the "correct" value. And then as soon as I started running again, they shot back up.

After the first mile, the HR readings settled down and were pretty accurate, but of course the average HR value was skewed.

I've also tried a different chest strap, and I get the same issues.

So in summary then, it looks like you have to run for 10-15 mins before your run to get correct HR readings, which IMHO is rubbish.

Note Garmin also advised me in a support call to "warm up" for 10 mins before a run so that the FR can use the correct algorithm to measure HR, which I guess is their way of saying it doesn't work if you just run.

Actually my observation is different: polar H10 sometimes reports very low heart rate. It is cold here in upper 30s. I do wetting of the strap before run. However, extreme low heart rate still happens, not at the beginning. I do not use gel as I am afraid it may damage the strap. In the attached figure, during the run, it drops to 88 bpm which is impossible. Max is 153. Pace is blue line and the gray portion is elevation.