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Garmin Edge calorie computation??

Bob IrwinBob Irwin
in General Training Discussions

I just got an Edge 500 for my bike.  I used the garmin for power, distance, speed, etc.  I wore my polar 625 for heart rate.  The calorie computation is roughly half what my polar 625 calculated.  Any ideas why?  Maybe the garmin is converting joules to calories without consideration as to body size, heart rate, etc.  Any ideas?  Does anyone know if the 310xt calculates things the same way?

Comments

  • manconamancona
    Hey Bob, my guess is both the polar and the edge/310 are off as most watches do a horrible job at this as they make all kinds of assumptions about what the "average" person is. Kind of like how my scale (even in athlete mode) says my BF is 3-4% higher then I had it measured at.

    If you want a somewhat accruate estimate of calories from the bike, either look at the power-kcal field on the edge 500, or use the joules from WKO after you download the file to WKO+.
  • mgraffeomgraffeo

    Bob, I'm not 100% sure of how the systems work, but I think I'm pretty close.  The garmin should be calculating calories based off of joules, as you said.  If it is doing it the same way the PowerTap computer does, this is probably the most accurate number you can get. 

    The Polar unit calculates calories based on HR variability, and your "owncal" estimate.  I haven't used my Polar in a while, but I remember these numbers being pretty good on the run, and a bit suspect on the bike.

    When in doubt, go by the numbers from your power meter.  Within a couple of percent, it's as close as you can actually get to measuring the calorie output.  It's not dependent on your size, % body fat, fitness, or any other parameter. 

    Mike

  • Bob IrwinBob Irwin

    The calories from the garmin were equal to the KJ for the ride - exactly.  The polar was roughly double.  I am pretty sure that the calories on the garmin are equal to the amount of work I did pushing on the pedals (I'm sure the PT measures that correctly), but I guess I'm not sure if that's how many calories my body used doing that work.  Since it came out to about 350 calories per hour, I think I'm iclined to thing that I used more.  I certainly couldn't have ridden much harder for most of the ride (4x(5x30/30) plus about 2 hours at 80-85% of FTP).

  • mgraffeomgraffeo

    Take a look at this calculation.  The full details are here. 

    If the calories were equal to the Kjoules, then they're using the above calculation.  Most people's efficiency falls somewhere between 22% - 25%, so divide by 4 then multiply by 4.  I'd go with the number on the Garmin.

    Mike

  • Bob IrwinBob Irwin

    Mike - that makes sense.  So the variable is really my personal efficiency.   It doesn't paint a pretty picture for people trying to lose weight.  If you'd always believed the published cal/hr for exercies (say 600-800cal/hr for a relatively intense bike), but are actually burning only half of that, then you'e got a problem. 

  • mgraffeomgraffeo

    Agreed, that's the biggest difficulty with those numbers you see on exercycles in health clubs.

    The easiest way to get that number of calories up?  Do the work we do here, increase your FTP, and push more watts.  More watts = more calories.  It's a bit cruel, but dudes like The Halligan burn crazy more calories in a workout than you or I do. 

    Mike

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