Variable HRM burns for the same exercise?

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I know that HRMs are not meant to be 100% accurate, but I just noticed how variable the burns can be.

I cycled 20 miles in 64 mins three times (indoor bike) within a 15 hrs. The bike gave the same burn (within a couple of cals) each time - but my HRM came up with 675, 785 and 883 cals. I expect thats because the HRM bases its burns on heart rate, and as I was getting more tired, I had to "try harder" to maintain the same speed (avg hear rate was higher with each attempt).

Is this "right"? Does one really burn more calories doing the same thing because one is tired? Or is that just the HRM's built in assumption that if your heart is beating faster, you must be doing more

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  • cwolfman13
    cwolfman13 Posts: 41,874 Member
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    I would say it's an assumption...one thing I learned with HRMs is that if you have anything out of the ordinary going on with your heart...high resting HR (that would result in higher HR while exercising) or low resting HR, etc...anything out of the ordinary will throw off the algorithm because your burn is not predicated on your actual HR...the HRM is simply using your HR in an algorithm that assumes a lot of statistical averages and is estimating that you're working at some % of your VO2 max and thus must be burning around X calories. But really, I was only burning a fraction of what my HRM told me because I had a resting HR of around 105 at the time which substantially inflated my burn...I guess the same could be for being tired and having your heart pump a little harder to make up for it.

    Just a guess though...
  • FrnkLft
    FrnkLft Posts: 1,821 Member
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    Yeah I'm glad you noticed that the HRM makes assumptions based on the HR it detects. You could get the thing to say you burned 100 calories just by holding your breath for a while (this also elevates heartrate).

    If the bike is not detecting your heart rate, then it's also making assumptions about power output and all that based on RPM, distance, and time.

    I would go with your HRM I guess. Things like intensity definitely affect your calorie burn, and so if you're not pushing quite as hard in the later exercises, then it would make sense that you burned less. Just think of the calorie difference between walking and running... the more you walk, less cal burn, the more you run, more cal burn. Just apply that to cycling.

    EDIT: Oh! And one more thing. Depending on which HRM you bought, the algorithm it's using to calculate calories burned might be based on running or cycling. It's possible that it's assuming you're running and that's throwing it off.