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HR Spikes

Howdy. Recently, I have been noticing that the HR data on my 935 seems to spike for no apparent reason. I am not going to say that I'm an ultra-runner or anything but I'm not a sofa potato either. After getting high HR warnings on my last run, I decided to pull out my trusty old FR 230 and strap to compare. During the run, I noticed that the 935 was indicating a bit higher on the HR but after stopping, both devices dropped to nearly the exact same number during my cool down.

However, during the walk to and from the park, I saw spikes in the 935 that were 40+ BPM higher than the 930. Often while I was simply walking at a leisurely pace or standing still. The timing of the devices was within about 3 seconds for a 5k run and the overall HR was surprisingly similar but these spikes never used to occur and I'm wondering how accurate the wrist HR really is (I understand it's not as good as the strap).

Anyone see anything similar?

Thanks.

0 mcalista over 7 years ago

Problems with high OHR readings are normally due to "cadence lock" - ie. the watch bouncing on your wrist with each swing of the arms, allowing external light to enter the sensor in a rythmic way. To avoid this, the watch should be on the fleshy part of the wrist well away from the wrist bone (this is higher than most people normally wear a watch), and the band needs to be fairly snug. I find I have to have the band one notch tighter for running than I do for 24/7 monitoring, and any time I notice a higher-than-expected HR, it's almost always a case of "yeah, forgot to tighten the band".

Cold weather can also affect things - not so much the watch itself, but because one of the body's responses to cold conditions is to restrict blood flow away from the extremities and the surface of the skin and preserve body heat for the vital organs in the torso and head, which makes OHR detection of changes in blood flow harder.

Of course, there isn't any reason why you can't use your existing HR strap with the 935.

0 Former Member over 7 years ago

"cadence lock" - ie. the watch bouncing on your wrist with each swing of the arms, allowing external light to enter the sensor in a rythmic way.

Pah, urban legend. Hold your arm with watch against *day* light and see what happens. Let alone how much of 530nm beams the sensor would/could need to catch if arms swing around in order to spike up.
If at all than it's a compromise of soothing artefacts vs. speed of the processing algorithm; or the lack of it.

0 mcalista over 7 years ago

Well, I don't know about you, but when I run with the watch loose on my arm, it reads consistently high. If I tighten the band, HR goes back to where it should be.

Even in good conditions, the amount of light that is reflected back into the sensor from beneath the surface of the skin is extremely low. It wouldn't take much additional light entering the region to be picked up - particularly if the additional light source is strobing at a frequency close to that of a typical heart rate. Typical running cadence is around 160-185 spm, which is probably slightly higher than the HR most people run at, but definitely in the feasible range, so can't be rejected by the algorithm.

0 Puddles808 over 7 years ago

Thanks for the replies Mcalista. I do tighten it down a notch for my runs but had never compared the spikes with the 230 and strap. The issue is that I was only walking to the park and was seeing these issues. During my run, it was pretty much spot on compared with the strap.