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HRM Tri - Frequency and Accuracy of readings on Garmin Connect

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  • HRM Tri - Frequency and Accuracy of readings on Garmin Connect

    Investigation into the potential causes of a fall from my bike...

    Hi - I put this originally in the FR935 subforum as I couldn't find an HRM-Tri specific forum. This might be a better place for it...

    I'm, interested in the second to second accuracy of the readings logged from the HRM, in this case, relating to a bike ride. I find that the HRM provides what I consider to be quite accurate tracks of my heart rate during bike rides - for example, when my speed drops whilst waiting at traffic lights I can see the Heart Rate dropping accordingly and often reassuringly quickly.

    Last week I had a fall from my bike - no traffic, no other riders, just me and, I thought, a clumsy acceleration after a brief rest to refill my water bottle. I have no recollection of the fall as I hit my head very hard and knocked myself out. Various studies at the hospital and GP afterwards indicate that I might have had a blackout due to a heart block although I remain sceptical.

    The heart would need to stopped for a few seconds to cause a blackout so I wondered if such a drop in heart rate would register on the readings sent from the HRM Tri. Would a zero reading for a few seconds show as zero (or very low) or is the tracked reading averaged for rates taken from the the previous 10, 20 or 60 seconds or something?

    I can see no noticeable drop in Heart Rate in the Garmin Connect logs immediately before the fall - only after. From the clip below you can see the restart of the ride, an acceleration to nearly 28kms which is quicker than I though I was going, and then a quick drop in speed (obviously), subsequent lowering in HR and a spike in cadence presumably as my pedals spun while I fell (pretty sure I went over the bars with my feet still slipped in).

    But there's no noticeable blip on the heart rate - should I expect to see one if my heart actually stopped or missed a few beats?

  • #2
    Why don't you make a simple experiment? Wear your HR strap and observe your HR on the watch. Then remove the strap from your chest and making sure that nothing touches the electrodes keep it removed for a couple of seconds. Then put your strap back on. This would tell you how the data behaves when your heart stops beating for a couple of seconds.
    Last edited by tmk2; 11-10-2017, 02:14 AM. Reason: Spelling and style.
    Current: nuvi 255, eTrex 30, Forerunner 310XT, Forerunner 735XT, HRM-Tri, HRM-Swim, SDM4, Virb Elite
    Retired: eTrex Legend, Swim


    • #3
      Yes, thanks, not sure why I didn't think that although I'd still like some insight into the technical method of the data logging.

      I did try as you suggested and the experiment showed that when removing the strap, which was 6:10, for about 20 seconds, the HR remained about constant at what it was when the strap was removed and then dropped gradually for another 40 seconds, then dropped immediately to zero. I put it back on and took it off at about 8:30 minutes with a similar result although this time it remained constant for 20 seconds and then dropped to zero.

      This seems strange to me - if the reading was aggregated over a period of time, then the drop to zero would have been more gradual, if it is supposed to be an immediate, low latency reading, then it would have dropped immediately when taken off, so the possibility is that the logging is delayed by about 20 or more seconds or the moisture in the sensors somehow tricks the readings that there is there is some latent HR still coming through.

      Either way, like speaking to most doctors about most things, the results still seem a little inconclusive.

      And I really want to know...