## / Calorie Counting

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Having logged a few rides using different gpx logging apps, I am interested in how they calculate calories used.

Most seem to use some combination of body weight and distance travelled. Sportstracker seems to take no account of time taken. So if I pootle round one of my circuits, it says I used the same number of calories as arriving at the end a sweating exhausted wreck in half the time.

At one time Sportstracker's calculation seemed wildly over the top. If I ate the same number of calories it reckoned I had used, I would have piled on a few stone per month.

I realize that a number of factors affect how many calories you use on a ride, but am interested in the general algorithms used.

Discuss!

Using Garmin 305 watch, it isn't accurate. One week I ran a couple of 10k loops, one was a few minutes quicker (+ higher av. HR) than the other and it said I'd burnt more calories on the slower one?...

I wouldn't bother with them!
In reply to wilkesley: I think they're only ever a 'guesstimate' at best, but I've used Strava and an app called 'My Fitness Pal' and they both come out with pretty similar values (although I suspect they overestimate slightly). One thing they can never account for is if you're riding in a group or on your own which makes a huge difference for a given speed.

> Most seem to use some combination of body weight and distance travelled. Sportstracker seems to take no account of time taken. So if I pootle round one of my circuits, it says I used the same number of calories as arriving at the end a sweating exhausted wreck in half the time.
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Not trying to be condescending but theoretically time doesn't make any difference:

Calories are a measure of energy or work. To propel something of a set mass a set distance takes the same amount of energy. If you do the work in a quicker time then you have achieved a higher power (energy divided by time) but the total energy used remains the same. This is pretty much the calculation these machines make.

In reality your body isn't 100% efficient and the hotter it gets the less efficient it is so going faster will use a bit more energy to do the same amount of work.
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> Using Garmin 305 watch, it isn't accurate. One week I ran a couple of 10k loops, one was a few minutes quicker (+ higher av. HR) than the other and it said I'd burnt more calories on the slower one?...
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> I wouldn't bother with them!

Im no expert but isnt this the way its supposed to work. The HR monitors actually slow you down to get the optimum burn rate.

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> In reality your body isn't 100% efficient and the hotter it gets the less efficient it is so going faster will use a bit more energy to do the same amount of work.

I imagine there is a certain level where your body is working at its most efficient. If you try and increase your output beyond that level I would guess that your energy expenditure would rise faster than your output.

I find calorie estimates useful as a guide as to whether I can eat one or two whole chocolate cakes without putting on weight Seriously, it is useful to be able to calculate calories burned as an overall plan to maintain or lose weight.

The most efficient your body is working could be fast, more to the point though, the differences will only be marginal.

I use the same calories running 10 miles as I do walking 10 miles, just the walking takes three times as long.
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> Not trying to be condescending but theoretically time doesn't make any difference:
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> Calories are a measure of energy or work. To propel something of a set mass a set distance takes the same amount of energy. If you do the work in a quicker time then you have achieved a higher power (energy divided by time) but the total energy used remains the same. This is pretty much the calculation these machines make.
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> In reality your body isn't 100% efficient and the hotter it gets the less efficient it is so going faster will use a bit more energy to do the same amount of work.

I don't believe it is as simple as that. Respiration requires energy, and you respire more with more vigorous exercise. Plus you have to factor in the increased work required to oppose wind resistance at higher speeds.

Consider a car as an example. It is more fuel efficient to travel 100 miles at 50mph than it is at 100mph.
In reply to Swirly: You are correct that calories are a measure or energy, but your assertion that time makes no difference is only true in certain circumstances.

Moving a mass a certain distance actually requires no energy at all because work = force*distance. If the mass is moving at a particular speed and requires no force to continue moving at that speed then no energy will be consumed no matter how far it moves.

If however you are going vertically upwards, then you must apply force to overcome gravity. To maintain a steady speed you must exactly match the force of gravity, so the force is m*g and the energy is just m*g*h where h is the vertical distance moved. In this situation, the distance is indeed the only variable.

However, the energy you lose cycling up a hill, you get back by descending it (kind of, obviously it doesn't refill your stomach, but if does give you 'free' speed). Most of the energy you actually expend on a bike is lost to air resistance. Air resistance is greater the faster you travel, but rather tha being proportional to speed it is proportional to speed squared. This means that if you travel a fixed distance very fast, you will lose more energy than someone travelling the same distance very slowly. Therefore calories are critically dependent on speed AND distance.