Garmin Edge - accurate calories?

Benjo74

So, I ride a 40 minute, 8 mile each way commute to work and according to the Garmin Edge 800, this burns off a nice 320 or so calories. Enough to justify a bacon butty when I get to work.

I went out on a MTB night ride this week, and we rode between 1.5 and 2 hours, and covered about 10 miles. Admittedly the distance wasn't much, but it was very hard going with all the mud and pools of water after the crappy recent weather. I for one was farked.

I checked the Garmin at the end of the ride, expecting to have "earned" a nice 800 or so calories given the state I was in after the ride, and was surprised/disappointed to see that it said I'd only burned about 400 calories.

I'm guessing that the calorie counter is only based on miles, by this logic, as opposed to recognising extra effort on hills and terrain? Anyone know? Anyone care?

cooldad

Nope. No one should need an excuse to eat bacons.

ddraver

Counting calories is phenomonally imprecise without extensive equipment and science wizardry. In reality it's a gimmick, I would nt pay it much attention tbh

njee20

The Edge 800 is marginally better than the Edge 705 (which is laughable), but IIRC just bases it purely on distance/speed, rather than factoring in any of the other information on hand - mine (705) ignores HR and power data which it could use to have a slightly more accurate guess!

The Polar algorithm is meant to be the most accurate, always seemed to me to give plausible guesses.

But yes, it's a completely useless training metric! That said, however muddy it was, 10 miles in 2 hours is rubbish ;-)

pilch

yes the accuracy is pants, I once burnt a few hundred calories on a ski lift

And got a KOM on Strava

bennett_346

Fail to see how any computer program can calculate the calories you have burned simply by your gps data...

dmorton

Work done = force x distance.

That will most likely be the basis of the energy calculation with some tweaks....

Also thinking I've burnt 400 calories therefore I can indulge 400 calories is flawed thinking. Calories don't take into account the different fat contents of food, how fast it will be metabolised etc. Glycemic Index is a better indication of a food's energy content, low = slow release, high=fast release. Balanced diet is the key, everything in moderation. So having a bacon sandwich after every ride to work probably isn't best, but once in a while is fine

Also there is evidence that you can burn as many calories after exercise than during it. http://www.nytimes.com/2011/04/19/health/nutrition/19best.html?_r=0

My girlfriend is type 1 diabetic and more than reckons this to be true. When exercising she's unlikely to go low blood sugar, it's afterwards (for quite a few hours) when she has to monitor blood sugar closely and finds a "hypo" more likely.

Benjo74

ddraver wrote:

In reality it's a gimmick

I'm a cyclist - I love gimmicks, gadgets and other shiny impractical and unnecessary things that tempt me to part with my hard earned!

dmorton

pilch wrote:

I once burnt a few hundred calories on a ski lift

Yup because by using GPS it knew you were moving, but it didn't know you weren't doing the moving (it was the lift)

njee20

I suspect he was well aware of that...

dmorton

njee20 wrote:

I suspect he was well aware of that...

........you never know. His best friend is a fish...look ^

bennett_346

dmorton wrote:

Work done = force x distance.

That will most likely be the basis of the energy calculation with some tweaks....

And how does one measure the force?

The Northern Monkey

bennett_346 wrote:

dmorton wrote:

Work done = force x distance.

That will most likely be the basis of the energy calculation with some tweaks....

And how does one measure the force?

midichlorians

davem399

I did a hard ride in the snow the other week, took an hour and did 5 miles off road. I was well knackered ploughing through several inches of snow, but my Garmin said I only burnt off 76 calories. I'm sure it thinks that 5 miles at a snails pace is easy, I beg to differ in this instance!

bennett_346

The Northern Monkey wrote:

bennett_346 wrote:

dmorton wrote:

Work done = force x distance.

That will most likely be the basis of the energy calculation with some tweaks....

And how does one measure the force?

midichlorians

I don't know what's worse, the fact you said that or the fact that i understood it

dmorton

bennett_346 wrote:

And how does one measure the force?

You calculate it from

Force = mass × acceleration

camerauk

Are you using a heart rate monitor with it as that may help it give a more accurate measurement with the calories at it will know that you are working hard ie when your heart rate is high when plugging through the mud etc

Briggo

pilch wrote:

yes the accuracy is pants, I once burnt a few hundred calories on a ski lift:

Er, face palm?

cooldad

Briggo wrote:

pilch wrote:

yes the accuracy is pants, I once burnt a few hundred calories on a ski lift:

Er, face palm?

Er face palm yourself, or have you really regressed so far you actually need a smiley to explain?

Briggo

cooldad wrote:

Briggo wrote:

pilch wrote:

yes the accuracy is pants, I once burnt a few hundred calories on a ski lift:

Er, face palm?

Er face palm yourself, or have you really regressed so far you actually need a smiley to explain?

Calm down old man, with the level of intelligence among some it can be deemed quite plausible that he had a blonde moment, yourself can be included within this at times.

Especially as there was lack of smiley or winky eye to say otherwise which is the universal code, I was merely questioning it, hence the oh you know, that wonderful thing called a question mark.

cooldad

Briggo wrote:

cooldad wrote:

Briggo wrote:

pilch wrote:

yes the accuracy is pants, I once burnt a few hundred calories on a ski lift:

Er, face palm?

Er face palm yourself, or have you really regressed so far you actually need a smiley to explain?

Calm down old man, with the level of intelligence among some it can be deemed quite plausible that he had a blonde moment, yourself can be included within this at times.

Especially as there was lack of smiley or winky eye to say otherwise which is the universal code, I was merely questioning it, hence the oh you know, that wonderful thing called a question mark.

Smileys are for people who have trouble counting to potato - see my sig.
You may note that I included a question mark as well, as I was giving you the benefit of the doubt. I'm kind that way.

njee20

That shouldn't really matter, calories consumed is a function of how hard you're working not purely how fast you're moving. Those two are generally (but not inextricably) correlated. If it knows how hard you're working (ie your power/hr) then the speed is a bit of an irrelevance. You don't burn more calories rolling down a hill than you do climbing it.

Comments about it being about 'distance moved' are flawed, because you're mechanically assisted on a bike.

pilch

dmorton wrote:

pilch wrote:

I once burnt a few hundred calories on a ski lift

Yup because by using GPS it knew you were moving, but it didn't know you weren't doing the moving (it was the lift)

Check out the big brain on Brett...

dmorton

njee20 wrote:

Comments about it being about 'distance moved' are flawed, because you're mechanically assisted on a bike.

Not at all. That would give the total work done by the system of the bike and you. Then you can get to your energy input by accounting for the mechanical advantage the bike gives you. You'd have to use an average for this, as the GPS tracker can't know what gear you're in, what traction you have, the friction in the drive system, overall efficiency etc.

This is how it appears that endomondo calculates it. If you edit a tracked ride's activity type from Mountain Biking to Fitness Walking, your calories consumed will go up (only slightly though). Change the activity type to Cycling, Sport (i.e. road biking) and it goes down. So it will be using distance moved, then tweaking it for the activity type based on an efficiency assumption.

https://getsatisfaction.com/endomondo/topics/calories_calculation_method

Benjo74

I'm not all that bothered about absolute accuracy, and partly it is all for a bit of fun or out of curiosity. I started monitoring the calories because I was trying to lose a bit of weight, and I got myself the Myfitnesspal app on my iphone which helps you do this by tracking calories burned through exercise along with your intake. It lets me scan the barcodes on my food and adds it to my daily tally. When I commute on the bike, I add the exercise and the Garmin calories to the app, which sets it off against my intake allowance for the day.
When I did the offroad ride and was knackered then saw that it was barely more than my commute I felt that I had earned more - just not sure how much. Of course, I know that it is all about the effort etc, and realistically I obviously burned more when climbing and spinning in the mud, but I was just wondering how much more, as a rough rule of thumb.

At the end of the day, it isn't all that importat to be accurate, but it has helped me eat better in moderation, and has encouraged me to get on the bike on days when I might typically have decided not to bother (looks like rain, looks a bit cold...).

As a result, I have lost a few kilos and I definitely have reduced the snacking and eating rubbish. Working in an office where people quite often bring in the Krispy Kreme selection boxes, the old me could have very easily polished off 2 or even 3 (late afternoon, there's some left, may as well have another, will only go to waste, etc...). Knowing how much of my daily allowance is in them, I am reluctant to now even eat one. Unless I get back on the bike and can "offset" the lard in my app with the exercise.

Anyway, thanks all for the comments - I reckon a fair measure is to add 50%-75% on top of the road ride calc, to get an approximation of the off road benefits. But it really doesn't matter that much, as long as I'm eating more sensibly and exercising more frequently.

And yes, before someone whinges, I guess this should have been posted in Fitness/Training. But it wasn't. :roll:

njee20

Not at all. That would give the total work done by the system of the bike and you. Then you can get to your energy input by accounting for the mechanical advantage the bike gives you. You'd have to use an average for this, as the GPS tracker can't know what gear you're in, what traction you have, the friction in the drive system, overall efficiency etc.

I'm not talking about Endomondo, just saying that you cannot directly correlate distance moved and calories used. The Garmin doesn't know how you're travelling - rolling down a hill on a bike uses far fewer calories than running down the same hill. The Garmin doesn't know this.

dmorton

njee20 wrote:

just saying that you cannot directly correlate distance moved and calories used. The Garmin doesn't know how you're travelling - rolling down a hill on a bike uses far fewer calories than running down the same hill.

Ah ok, so the Garmin doesn't ask what your activity is?

njee20

No, and even when you provide it with data like power - so it knows exactly how hard you're working, it ignores it, and bases the figure on speed/distance alone.

dmorton

http://www.dcrainmaker.com/2010/11/how-calorie-measurement-works-on-garmin.html

Number 5 on this link is the speed/distance calculation, i.e. what Benjo74 was using. They rate it as 65-80% accurate. Does the Garmin ask for your weight? Regardless, the fundamental basis of the maths will be Work done = force x distance moved.

Garmin seems they ignore vertical distance for that calculation and just go with horizontal, because GPS elevation calculations is deemed not accurate enough. Therefore, even if it asks for it, the difference your weight makes probably isn't accounted for either.

njee20

The older Edge units are different to the newer ones. Mine, with barometric altimeter, reckons I can do 1500 calories in an hour of fairly steady cycling, and it knows my (70kg) weight.

Benjo74

dmorton wrote:

http://www.dcrainmaker.com/2010/11/how-calorie-measurement-works-on-garmin.html

Does the Garmin ask for your weight?

Yes - it knows my age, weight, approximately how active I am, and even the weight of the bike. If I'm on the roadie, it even knows my cadence, but I didn't bother with a heart rate monitor. But, all that info might not be used by the Garmin when giving a calorie count. It could well just be distance travelled and time taken. Like someone else said, I could probably burn off a few bacons by just coasting down a hill for a bit.