I am trying to use HR as a guide for run intensity and for my two runs on the "100" challenge I've had very different results. The first run the HR was reading a false HIGH of 160 bpm. On my second run using the treadmill the HR was working fine.
I found this explanation online which is exactly what is going on with my watch. Posting as a reference for others. My watch is a TomTom Spark3 and uses the typical optical sensor:
"Spikes in HR are generally from poor blood flow producing weak pulse strength, so the watch reads cadence instead. This is most common in running and is particularly apparent early in a workout or during a non-intense workout when you are not warmed up or when you are doing sprints with very high effort.
You have to think of the optical heart rate as an algorithm that is attempting to track a signal in a set frequency range (30-230 or whatever it uses). If the pulse signal is weak it latches onto the next strongest rhythmic signal, which is your cadence in running and the vibrations of the bike in cycling. For most people who experience this while running it spikes to around 180-200 bpm which is also the average cadence people run at.
Additionally, each person has a different HR signal ‘strength’, depending on a range of factors, so some are prone to get it more than others. But usually their signal strength is lower for the first 5-10 minutes until they warm up properly. So in that time, it is prone to latching onto cadence, which is a common fault with all optical HRs, unfortunately. Optical HR also can tend to lag in measurements, so if you are doing intervals it can take a but to catch up so it shows high HR during the rest periods. If you notice it while it happening you can try moving the watch a bit or briefly pausing your run so it loses the cadence reading and latches back onto HR, which I find usually corrects it. I generally pause the watch, stand still for 20-30 seconds and will see it immediately start to drop. Once it gets into a more reasonable range and the pulse reading stops dithering (dithering is when it is not getting a good signal and it is a lighter grey in color) I start up again and it stays true for the rest of the run. You can also try switching wrists and the position on the wrist. I find I got better readings on my right wrist over my left and some people find they get better readings if the watch is on the inside of the wrist rather than the outside. It also helps if you warm up a bit to get your blood moving and your HR up so it is producing a strong signal. Play around with it and see if any of this helps you. The challenge for the manufacturers of optical HRs (and this is a common issue with all brands, my Scosche also does it) is to figure out how to factor out the other "noise" that is overriding the pulse signal without also factoring out other important data.
I hope this helped answer your question. If so, please mark it as a solution so others can look for it if they have the same question."
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