I don't cycle enough to justify power meters, but likely even if I did I wouldn't buy them. In the end it most of this fitness related stuff is down to best estimates, and adjust as needed. Even if a person manages to buy very accurate meters, the gross cost still isn't an exact.

Unless they have Bill Gates money, no one buys power meters to count calories. That's not what they're for. Being able to tell you how many calories you've burned is an interesting side effect of how they work, but not worth $300+ for most people. I'm getting this weird feeling that some MFP Mythology is being created in this thread. 🙂 Power meters are a training tool, they're useful because they measure intensity in a repeatable and precise way. The data they produce has other uses including frankly astonishing calorie accuracy. Analogy: a car has lots of uses but nobody buys one to store things on the back seat. 🙂

I cycle, which brought me to this topic.

The only thing I'm smart enough to take exception to is the idea that no one buys a car to store things in the back seat. That person obviously hasn't seen my wife's car (Or the trunk of my car as I use it for work...)

Carry on with the more serious aspects of the discussion.

I've just changed from a Garmin Edge to a Wahoo Elemnt Roam this week and was doing some reading on their partner sites and came across this on Sufferfest.....

"Why are kJ and kCal used interchangeably at times?

We can say that 1 kcal = 1kJ when talking about riding bikes due to the efficiency of humans. When converting food energy into movement (like pedalling), humans are only around 24% efficient. For every 1kJ of work completed externally (pedalling) requires around 4kcal of energy (or about 16.7 kJ of energy). The rest of that energy is released as heat, which is why exercise makes you sweat; your core body temperature is increasing. To put the heat production in another context, riding at 250W creates roughly 750W of heat, which is the same as most toasters.

Now not everyone is 24% efficient. The actual conversion rates we have measured in our lab range from 0.9 kJ = 1 Kcal up to 1.12kJ = 1 kcal.
So using this 1:1 formula will still under and overestimate calories for people, but it is significantly more accurate and reliable than the heart rate based calculations."

Do keep in mind that a 10% difference in efficiency doesn't result in a 10% difference in calories, it's 10% of 24% so a net calorie range of plus or minus 2.4%.
If someone's typical ride is 600cals that a range of 586 - 614cals, I'd say that is more than accurate enough for the purpose of calorie counting.

1 cal(th) = 4.184 J (Exactly by definition)
1 kcal(th) =4.184 kJ (Exactly by definition)

Food "calories" are actually kcals(th). It just happens that your "gross efficiency" is pretty close to 1/(4.184) = 23.9%, so the two factors nearly cancel. Sweet! (I think this is why this particular value has been adopted so universally: it makes the math easy.)

Also, a J (joule) is a W-s (a watt-second). So, if you can pull 180W for an hour, you output 180 * 3600/1000 = 648kJ = 155kCal (output). To get your approximate calories burned in doing so, you divide by the gross efficiency, ~0.239, to get 684kCals (consumed).

As to @sijomial 's statement about how the error propagates, I think you should calculate a couple of cases. If your gross efficiency is off by 10%, then your consumed calorie estimate will also be off by 10%. I've now read 3 different peer-reviewed papers where they have directly measured the gross efficiency, and I see that it can easily vary by more than 10% between individuals.

If you need more proof, search at https://pubmed.ncbi.nlm.nih.gov/ using the terms "cycling gross efficiency." You'll find ~466 citations, because it's very easy to measure. How it's measured is an entirely different story!

1 cal(th) = 4.184 J (Exactly by definition)
1 kcal(th) =4.184 kJ (Exactly by definition)

Food "calories" are actually kcals(th). It just happens that your "gross efficiency" is pretty close to 1/(4.184) = 23.9%, so the two factors nearly cancel. Sweet! (I think this is why this particular value has been adopted so universally: it makes the math easy.)

Also, a J (joule) is a W-s (a watt-second). So, if you can pull 180W for an hour, you output 180 * 3600/1000 = 648kJ = 155kCal (output). To get your approximate calories burned in doing so, you divide by the gross efficiency, ~0.239, to get 684kCals (consumed).

As to @sijomial 's statement about how the error propagates, I think you should calculate a couple of cases. If your gross efficiency is off by 10%, then your consumed calorie estimate will also be off by 10%. I've now read 3 different peer-reviewed papers where they have directly measured the gross efficiency, and I see that it can easily vary by more than 10% between individuals.

If you need more proof, search at https://pubmed.ncbi.nlm.nih.gov/ using the terms "cycling gross efficiency." You'll find ~466 citations, because it's very easy to measure. How it's measured is an entirely different story!

It may come as a shock to realize these interesting facts you've just learned from the google have been well known for decades. 🙂 No you don't need to point the basics out, but it may be instructive:

I think I see where things are going wrong, you aren't understanding effeciency. In your example you divided by four an extra time. 😉 If you're putting 180w into the road you're making 540w of waste heat.

@NorthCascades Have I made any progress in explaining the issue? You point out that my last post was obvious, but you also seem to think there is an error. In what way am I misinterpreting the gross efficiency? Is it not the ratio of the energy applied to pedals to the total energy expended by the human body?

(I think the fact that one is measured in kJ and the other is measured in kCal can be a bit confusing. Also, that the unit conversion and the gross efficiency are about the same magnitude is also confusing.)

When it comes to the efficiency argument, I look at it this way. The power meter only measures power being applied to move th pedals. That doesn't necessarily mean there isn't energy being expended that isn't going towards the pedal rotation. For example, what about pedals with clips on them. They allow to use energy in lifting your foot during the 6 - 12 part of the pedal stroke and apply it to move the bike.

## Replies

852Member Member, Premium Posts:852MemberI cycle, which brought me to this topic.

The only thing I'm smart enough to take exception to is the idea that no one buys a car to store things in the back seat. That person obviously hasn't seen my wife's car (Or the trunk of my car as I use it for work...)

Carry on with the more serious aspects of the discussion.

18,528Member Member, Premium Posts:18,528Member"Why are kJ and kCal used interchangeably at times?

We can say that 1 kcal = 1kJ when talking about riding bikes due to the efficiency of humans. When converting food energy into movement (like pedalling), humans are only around 24% efficient. For every 1kJ of work completed externally (pedalling) requires around 4kcal of energy (or about 16.7 kJ of energy). The rest of that energy is released as heat, which is why exercise makes you sweat; your core body temperature is increasing. To put the heat production in another context, riding at 250W creates roughly 750W of heat, which is the same as most toasters.

Now not everyone is 24% efficient. The actual conversion rates we have measured in our lab range from 0.9 kJ = 1 Kcal up to 1.12kJ = 1 kcal.

So using this 1:1 formula will still under and overestimate calories for people, but it is significantly more accurate and reliable than the heart rate based calculations."

Do keep in mind that a 10% difference in efficiency doesn't result in a 10% difference in calories, it's 10% of 24% so a net calorie range of plus or minus 2.4%.If someone's typical ride is 600cals that a range of 586 - 614cals, I'd say that is more than accurate enough for the purpose of calorie counting.

https://support.thesufferfest.com/hc/en-us/articles/360015061719-Calorie-Calculations-

2,805Member Member, Premium Posts:2,805Member1 cal(th) = 4.184 J (Exactly by definition)

1 kcal(th) =4.184 kJ (Exactly by definition)

Food "calories" are actually kcals(th). It just happens that your "gross efficiency" is pretty close to 1/(4.184) = 23.9%, so the two factors nearly cancel. Sweet! (I think this is why this particular value has been adopted so universally: it makes the math easy.)

Also, a J (joule) is a W-s (a watt-second). So, if you can pull 180W for an hour, you output 180 * 3600/1000 = 648kJ = 155kCal (output). To get your approximate calories burned in doing so, you divide by the gross efficiency, ~0.239, to get 684kCals (consumed).

As to @sijomial 's statement about how the error propagates, I think you should calculate a couple of cases. If your gross efficiency is off by 10%, then your consumed calorie estimate will also be off by 10%. I've now read 3 different peer-reviewed papers where they have directly measured the gross efficiency, and I see that it can easily vary by more than 10% between individuals.

If you need more proof, search at https://pubmed.ncbi.nlm.nih.gov/ using the terms "cycling gross efficiency." You'll find ~466 citations, because it's very easy to measure. How it's measured is an entirely different story!

10,442Member Member Posts:10,442Member10,442Member Member Posts:10,442MemberIt may come as a shock to realize these interesting facts you've just learned from the google have been well known for decades. 🙂 No you don't need to point the basics out, but it may be instructive:

I think I see where things are going wrong, you aren't understanding effeciency. In your example you divided by four an extra time. 😉 If you're putting 180w into the road you're making 540w of waste heat.

2,805Member Member, Premium Posts:2,805Member(I think the fact that one is measured in kJ and the other is measured in kCal can be a bit confusing. Also, that the unit conversion and the gross efficiency are about the same magnitude is also confusing.)

968Member Member Posts:968Member