HR & calories

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HR & calories

Postby swaz » Sun Jun 26, 2011 11:37 am

I ride approx. 25km into work sometimes. I've put all the correct data into my Garmin and it tells me I burn about 1000cal one way. The trip takes me between 55mins to 1hr and my HR is average 145. I'm 29, unfit, 85kg.

Does this sound like a fair assessment by my 310xt??
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by BNA » Sun Jun 26, 2011 1:38 pm

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Re: HR & calories

Postby OutOfBreath » Sun Jun 26, 2011 1:38 pm

I'm also 29 and unfit. 91kgs.

On a 25km ride with an average heart rate between 150 and 160 my Garmin says that I burn about 950 calories.

I don't know if that's wrong, right, or even close... I don't use the numbers as gospel, but just as a general guide as to how much effort I have put in to the ride. I don't think you could rely on these devices to be all that accurate considering how different every person's body responds differently to exercise.


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Re: HR & calories

Postby Myddraal » Sun Jun 26, 2011 2:48 pm

Calorie counters aren't accurate at all, but if you're only using one you can use it as a general guide to how hard you've worked.
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Re: HR & calories

Postby swaz » Sun Jun 26, 2011 4:05 pm

Cheers for the replies. I am aware they are not greatly accurate. Ballpark is fine for me. Just slightly shocked that for my commute I would burn close to 2k calories.
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Re: HR & calories

Postby Alex Simmons/RST » Sun Jun 26, 2011 6:00 pm

swaz wrote:I ride approx. 25km into work sometimes. I've put all the correct data into my Garmin and it tells me I burn about 1000cal one way. The trip takes me between 55mins to 1hr and my HR is average 145. I'm 29, unfit, 85kg.

Does this sound like a fair assessment by my 310xt??

No. Assuming you mean 1000kcal and not 1000cal.

1000kcal would require you to maintain an average of ~280 watts on your commute, not something an unfit rider would typically be expected to do.
I'd suggest it's over estimating by nearly 100%
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Re: HR & calories

Postby ValleyForge » Sun Jun 26, 2011 10:41 pm

Garmin have improved the algorithm to calculate the energy expenditure for the 800 series. Unfortunately all the older models overestimate significantly. My saturday ride on my Garmin 705 is ~2700kcal, on my mates 800 is about 1800kcal. We're close to the same weight/bikeweight/style.

Alex is right - 280W would be what you need to output and the garmins default to kcal. Just think of it as being generous as to how many Big Macs you can eat afterward....
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Re: HR & calories

Postby hosko » Mon Jun 27, 2011 7:40 am

My Cateye says a lot less than that.

I am 32, trying to get fit, 109kg.

We do 22kms in about 48 mins twice a week during our lunch breaks at work.

When I wear my HR strap, I got the other day under 500 calories listed in the computer for that time. HR was averaging 170 according to the computer.

Not sure how accurate that is but we are stuffed when we get back as it is not all flat!
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Re: HR & calories

Postby lethoso » Mon Jun 27, 2011 10:22 pm

Alex Simmons/RST wrote:1000kcal would require you to maintain an average of ~280 watts on your commute, not something an unfit rider would typically be expected to do.
I'd suggest it's over estimating by nearly 100%


wouldn't that be 1000kJ of work done (~250 kcal)? Or are you assuming ~25% efficiency for a cyclist?
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Re: HR & calories

Postby open roader » Mon Jun 27, 2011 10:49 pm

My Sigma bike computer has a calorie burner function with my weight etc entered into the computer as a reference. I doubt that it's accurate per se but subjectively, I do find it consistant in the amount of calories it says I burned vs. the amount of effort I am conscious of putting in on any given ride. I'm no serious trainer but I do like to second guess the calorie counter just before the end of each ride and find my pre-determined estimates usually are close to what the computer says. This is as far as I'd consider a simple energy consumption function on a generalised bike computer to be of use.
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Re: HR & calories

Postby Alex Simmons/RST » Tue Jun 28, 2011 8:44 am

lethoso wrote:
Alex Simmons/RST wrote:1000kcal would require you to maintain an average of ~280 watts on your commute, not something an unfit rider would typically be expected to do.
I'd suggest it's over estimating by nearly 100%


wouldn't that be 1000kJ of work done (~250 kcal)? Or are you assuming ~25% efficiency for a cyclist?

As a rule of thumb, 1 kJ of mechanical work done at the cranks requires us to metabolise 1 kcal of energy.

That ratio can vary depending on gross metabolic efficiency but note that:
1 kj = 0.239 kcal
GME typically ~ 19-24%

So that pretty much means a 1:1 conversion. It's really more like 1.1 : 1 give or take .1, but for a quick estimate, it's good enough.
Certainly sufficient to know that the calorie counter used by the OP is way, way off in its estimate.
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Re: HR & calories

Postby lethoso » Tue Jun 28, 2011 10:06 am

Alex Simmons/RST wrote:That ratio can vary depending on gross metabolic efficiency but note that:
1 kj = 0.239 kcal
GME typically ~ 19-24%


cool! interesting to know, cheers :)
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Re: HR & calories

Postby BarryTas » Tue Jun 28, 2011 10:44 am

swaz - borrow a polar hr monitor or anyther brand and wear that on your commute and not the garman and compare the stats

make sure the info in the hr monitor are correct ie, sex, age, weight etc
when do we stop for coffee???

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Re: HR & calories

Postby Alex Simmons/RST » Tue Jun 28, 2011 11:30 am

BarryTas wrote:swaz - borrow a polar hr monitor or anyther brand and wear that on your commute and not the garman and compare the stats

make sure the info in the hr monitor are correct ie, sex, age, weight etc

OK so two riders same weight, sex, age. One has threshold power of 180W, the other 300W. Both have same HRmax & threshold HR.

How can the HRM provide a sensible calorie estimate for both riders?
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Re: HR & calories

Postby BarryTas » Tue Jun 28, 2011 12:06 pm

Alex Simmons/RST wrote:
BarryTas wrote:swaz - borrow a polar hr monitor or anyther brand and wear that on your commute and not the garman and compare the stats

make sure the info in the hr monitor are correct ie, sex, age, weight etc

OK so two riders same weight, sex, age. One has threshold power of 180W, the other 300W. Both have same HRmax & threshold HR.

How can the HRM provide a sensible calorie estimate for both riders?



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Re: HR & calories

Postby nickobec » Tue Jun 28, 2011 12:40 pm

The older Garmin (ie my 705) calories burnt algorithm is based purely on average speed, nothing to do with HR:

For example:
50 minutes at 36kmh avg HR 150 (group ride with tail wind) 1500 calories
2 hours at 20kmh avg HR 150 (return trip solo into headwind and then hills home) 1300 calories
70 minutes at 30kmh avg HR 115 (post heart attack, now with beta blockers, hence lower HR) 1600 calories
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Re: HR & calories

Postby Alex Simmons/RST » Tue Jun 28, 2011 2:10 pm

nickobec wrote:The older Garmin (ie my 705) calories burnt algorithm is based purely on average speed, nothing to do with HR:

For example:
50 minutes at 36kmh avg HR 150 (group ride with tail wind) 1500 calories
2 hours at 20kmh avg HR 150 (return trip solo into headwind and then hills home) 1300 calories
70 minutes at 30kmh avg HR 115 (post heart attack, now with beta blockers, hence lower HR) 1600 calories

Well that just goes to show that speed is just as bad.

1500cal/50-min!!!
That's 500 watts at the crank. :lol:
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Re: HR & calories

Postby toolonglegs » Tue Jun 28, 2011 2:46 pm

IIRC my Polar CS200 was "quite" accurate on full out hour efforts...overestimated a bit but not too much,maybe 10%.But it hugely underestimated on recovery / endurance type rides for me...my heart rate hardly moves till I actually start working pretty hard.I won't be bothering with a heart rate monitor ever again...maybe a powermeter again one day.
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Re: HR & calories

Postby ValleyForge » Wed Jun 29, 2011 9:46 am

toolonglegs wrote:IIRC my Polar CS200 was "quite" accurate on full out hour efforts...overestimated a bit but not too much,maybe 10%.But it hugely underestimated on recovery / endurance type rides for me...my heart rate hardly moves till I actually start working pretty hard.I won't be bothering with a heart rate monitor ever again...maybe a powermeter again one day.


Day to day I think an HR minitor is OK, but trying to extrapolate a whole lot of extra information from just HR & speed is wildly inaccurate as we can see.

And, yes, a powermeter one day..., but I'm a first-timer.
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Re: HR & calories

Postby swaz » Fri Jul 08, 2011 5:28 pm

Thanks for the feedback. Disappointed I am not burning as much as I thought.
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Re: HR & calories

Postby johnhunt » Sat Jul 09, 2011 10:20 pm

I have an HRM, it's interesting to see how far off the gym machines are from that too.. over 45 mins of exercise they seem to be at least 100kcals off. As others have said they're not really very accurate but they *are* good for benchmarking your own performance. Originally I found it encouraging, but now I find it just depresses me after a very serious workout only burning off 700 or 800kcals (and not be able to beat that no matter how hard I try (over an hour at the gym.))
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Re: HR & calories

Postby Alex Simmons/RST » Sun Jul 10, 2011 10:15 am

Let me make this quick comment:

The Calories you metabolise riding a bicycle can only be measured by knowing:
i. Volume of Oxygen utilisation (VO2)*, or
ii. Power output, duration and gross metabolic efficiency (GME can be determined from power output and O2 utilisation data)


* VO2 can only be measured using a metabolic cart in a lab
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Re: HR & calories

Postby leighthebee » Fri Jul 15, 2011 3:05 pm

Now I won't dispute the maths on this thread so far, as its far from my forte, but my experience has me similar to yours at burning around 1000 kCal per hour when I was unfit and working hard.

I have dropped that by about 25% since my average H/R has dropped as it ride more comfortably.....
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Re: HR & calories

Postby Alex Simmons/RST » Fri Jul 15, 2011 7:15 pm

leighthebee wrote:Now I won't dispute the maths on this thread so far, as its far from my forte, but my experience has me similar to yours at burning around 1000 kCal per hour when I was unfit and working hard.

I have dropped that by about 25% since my average H/R has dropped as it ride more comfortably.....

There in lies the problem of thinking HR has anything to do with calories metabolised.

Apart from 1000kcal/hr probably being wrong to start with, just because you are fitter and doing it more easily doesn't mean you are doing less work.
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