Watts to Calories vs Heart Rate to Calories

Bitokos
Bitokos Posts: 26 Member
in Fitness and Exercise
Hello All,

Just wondering about the different ways to calculate calories. I biked for 45 minutes this morning with an average HR of 163. That translated to about 720 calories based on using Polar Beat with the H7 HRM.

I was on a Kickr Snap using zwift and it said that I had an average wattage of 85. 85x0.75*3.6 = 230 Calories. Even if there was 25% efficiency loss that would only get it to 300 Calories.

Is the Polar Flow over estimating calories by almost double? I was totally beat when I was done. If I put moderate cycling in MFP I think it said around 500.

Thanks for any help or correction to math.

Replies

  • NorthCascades
    NorthCascades Posts: 10,968 Member
    If your Kickr Snap is even close to accurate, then the conversion from watts will be drastically better than from your heart rate.

    One is measuring your output, ie the work you did, the other is measuring your body's reaction to that.

    Your heart rate is affected by your emotional state, hydration status, amount of sleep, etc. Your power output is affected by the rate at which you're performing work.

    No question. It's almost like using a bathroom scale or a pair of dice to know how much you weigh.
  • MrWilson6
    MrWilson6 Posts: 148 Member
    Your Polar Beat is the more accurate calculation. The app will never be 100% accurate, but it's not far off. I am curious about your wattage though, with it only being 85. Do you mean cadence, because that would make more sense.
  • stealthq
    stealthq Posts: 4,298 Member
    Bitokos wrote: »
    Hello All,

    Just wondering about the different ways to calculate calories. I biked for 45 minutes this morning with an average HR of 163. That translated to about 720 calories based on using Polar Beat with the H7 HRM.

    I was on a Kickr Snap using zwift and it said that I had an average wattage of 85. 85x0.75*3.6 = 230 Calories. Even if there was 25% efficiency loss that would only get it to 300 Calories.

    Is the Polar Flow over estimating calories by almost double? I was totally beat when I was done. If I put moderate cycling in MFP I think it said around 500.

    Thanks for any help or correction to math.

    It's more than possible. HR isn't a great way to estimate calories burned. There's a correlation there, but it's indirect.

    HR is increased by all kinds of things beyond respiration needs. Temperature, stimulants (including caffeine in coffees and sodas), lower degree of cardio fitness, length of workout (see: cardiac drift), etc. In addition, you may be starting out with a higher range of heart rates than the monitor expects. If your max HR is higher than the standard 220-age and you didn't or can't calibrate your monitor to match, it will definitely calculate higher calorie burns than it should. Plus, it may be calculating gross calorie burns and not net. A lot of them do.

    I can't speak to the Snap - I'm not a cyclist and I'm not familiar with its accuracy.
  • sijomial
    sijomial Posts: 19,809 Member
    Are you sure your average was only 85 watts?
    That's very gentle cycle - are you sure you have read that display right as would be surprised you would get your HR up at that level.
    720 cals for 45 minutes would be a good effort for a reasonable cyclist but that certainly wouldn't be at 85 watts.
  • NorthCascades
    NorthCascades Posts: 10,968 Member
    Bitokos wrote: »
    I was on a Kickr Snap using zwift and it said that I had an average wattage of 85. 85x0.75*3.6 = 230 Calories. Even if there was 25% efficiency loss that would only get it to 300 Calories.

    You can ignore efficiency when you do the math this way, because 1 Cal = 4.184KJ.
  • Bitokos
    Bitokos Posts: 26 Member
    I was going all out and I am in pretty decent shape. I will try to figure out if I've hooked something up wrong. My HRM has really served me well when it comes to MFP.
  • BrianSharpe
    BrianSharpe Posts: 9,248 Member
    Do you have any way of estimating how far you rode in that 45 minutes?

    Like others have said 85 watts is a pretty gentle ride and there's no direct correlation between heart rate and energy expenditure . You could have two riders (one fit, one less so) do the same ride but a HRM would incorrectly estimate a higher energy expenditure from the less fit rider as a result of higher heart rate.
  • NorthCascades
    NorthCascades Posts: 10,968 Member
    For me personally, a recovery ride means < 100 w and < 100 bpm.

    On the other hand, if your FTP was 90 w, this would feel like a very difficult ride. Personally I doubt that's the case, but I don't have enough info to say for sure.

    It sounds like the Kickr is probably under-reporting. I don't know if it's supposed to report L/R Balance, if so it might be halving your power. Or maybe it doesn't actually measure power, but uses a curve to estimate it from your wheel speed?
  • Bitokos
    Bitokos Posts: 26 Member
    My FTP was 85 Watts. So, I guess I am using the information incorrectly. Which is why I came here. I just bought the kickr yesterday.
  • NorthCascades
    NorthCascades Posts: 10,968 Member
    Does it tell you the intensity factor (IF) was 1.0 and you earned a training stress score (TSS) of 75? If so, download some free software called Golden Cheetah. More on that later.

    When you get on the bathroom scale, it uses a clever device called a strain gauge to figure out what you weigh. You bend the scale by standing on it, the more you weigh the more it bends, so it measures that deformation to figure out your weight. It isn't estimating your weight, it's measuring it, maybe somewhat indirectly.

    Your Kickr has a strain gauge (maybe several) that measure how much torque you're applying to the bike, through the pedals and chain. It also measures how often you apply torque, in RPMs.

    Power = torque * rate
    1 watt = 1 Joule per second
    1 Joule = 1/4.184 Calorie

    You probably also ride outdoors, right?
  • Bitokos
    Bitokos Posts: 26 Member
    Yes, until my son was born 8 months ago. Now I have zero time for that ha ha. My wife "appreciates" me riding indoors so I can help out when needed instead of being miles away.
  • NorthCascades
    NorthCascades Posts: 10,968 Member
    So you haven't been on a bike in 8 months? Am I reading that right? If so, 85 watts is pretty reasonable and believable. If so, your Kickr probably works perfectly. (Most power meters do, but like anything, there are a few duds.)
  • kcjchang
    kcjchang Posts: 709 Member
    MrWilson6 wrote: »
    Your Polar Beat is the more accurate calculation. The app will never be 100% accurate, but it's not far off. I am curious about your wattage though, with it only being 85. Do you mean cadence, because that would make more sense.

    Estimate of expenditure of energy using HRM is more accurate than a power meter, you kidding right? Even at +/-5%, the kickr is going be night and day as far as accuracy. If you are referring to speed, I agree that zwift is slippery (no idea what CdA), but wattage is from the trainer. Or are you just referencing this particular referenced stats?

    OP, use the ride's total kilojoules to calculate the Calories expended.
  • Bitokos
    Bitokos Posts: 26 Member
    NorthCascades wrote: »
    So you haven't been on a bike in 8 months? Am I reading that right? If so, 85 watts is pretty reasonable and believable. If so, your Kickr probably works perfectly. (Most power meters do, but like anything, there are a few duds.)

    I did another ride this morning. It was the FTP Ride to calculate my FTP. My average watts was 131. My FTP was estimated to be 109. I am a bit out of shape and thought I was going to fall off my bike at the end. It completely wore me out before it got to the FTP portion. Which I guess is how that works. It had me cycling at 235, 280, and 325 Watts for 20 second bursts before the FTP portion.

    I am using Zwift. Does anyone else use that app?

    I really appreciate everyone's input.

    Thanks!
  • NorthCascades
    NorthCascades Posts: 10,968 Member
    About a year ago, I got hit by a car, and couldn't ride for months. My power output / FTP dropped like a rock. Stuff that used to be easy wiped me out. But it came back.

    It's pretty normal to see your output jump around almost like random. Instead of looking at my right now power, I've got my computer set up to see the average power for the last 3 seconds. It's kind of a compromise between immediate and less jumpy, because mine does that too.
  • SingingSingleTracker
    SingingSingleTracker Posts: 1,866 Member
    Bitokos wrote: »
    NorthCascades wrote: »
    So you haven't been on a bike in 8 months? Am I reading that right? If so, 85 watts is pretty reasonable and believable. If so, your Kickr probably works perfectly. (Most power meters do, but like anything, there are a few duds.)

    I did another ride this morning. It was the FTP Ride to calculate my FTP. My average watts was 131. My FTP was estimated to be 109. I am a bit out of shape and thought I was going to fall off my bike at the end. It completely wore me out before it got to the FTP portion. Which I guess is how that works. It had me cycling at 235, 280, and 325 Watts for 20 second bursts before the FTP portion.

    I am using Zwift. Does anyone else use that app?

    I really appreciate everyone's input.

    Thanks!

    You could also do a ramp test (MAP) based on your current conditioning. It's painful as well, but for less duration and the results will be very similar to the 20 minute FTP test.

    https://cyclingtips.com/2010/01/how-to-do-a-map-ramp-test/

    All the best with your training and progress.

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