Powertap energy reading

[bladerunner]

Hi,
I have a Cycleops Powertap meter and I've not taken much notice of the Energy-expenditure reading up to now. The manual says the reading is shown in "kilojoules". So, do I just divide the number by 4.2 to convert to Calories/Kcal?

 

[elapid]

When using a PT your energy expenditure = calories burned. PT calculates energy expended based on work done and estimates that we are about 25% efficient. So yes, to convert kJ to calories you divide by 4.2, but because PT estimates efficiency at 25% you then multiply by 4, so it all comes out in the wash as your energy expenditure = calories burned.

 

[bladerunner]

Hi, thanks for the quick response.
So, the number shown on the PowerTap meter is essentially the Kcal used. Right?

 

[elapid]

Yep it is.

 

[Alex Simmons/RST]

When using a PT your energy expenditure = calories burned. PT calculates energy expended based on work done and estimates that we are about 25% efficient. So yes, to convert kJ to calories you divide by 4.2, but because PT estimates efficiency at 25% you then multiply by 4, so it all comes out in the wash as your energy expenditure = calories burned.

This is incorrect. The PT makes no such assumption, nor estimates energy metabolised.

The Powertap records the mechanical work done at the rear hub = average power (watts) x time (seconds).

If you want to estimate the energy metabolised, then you divide the mechanical work done by your gross mechanical efficiency (GME - typically in the range of 18%-24%).

If you want to convert that to Calories (kcal), then divide again by 4.18 (kJ per kcal).

What your GME is depends on a number of things (it can be measured but requires lab testing).

 

[elapid]

Hi Alex,

Not wanting to be controversial, but I asked this question when I first started using a PT a few years ago and my CTS coach, CycleOps customer support, and the PT instruction manual all said the same thing. The energy expenditure on PT = calories for the reasons stated.

 

[Alex Simmons/RST]

Hi Alex,

Not wanting to be controversial, but I asked this question when I first started using a PT a few years ago and my CTS coach, CycleOps customer support, and the PT instruction manual all said the same thing. The energy expenditure on PT = calories for the reasons stated.

Then they were wrong. Nothing controversial about that.

The PT manual does not say that. This is what they (Powertap manuals from 2005 to 2011) actually say:


Energy Expenditure (E)
The total work done over the course of the trip or interval is shown in kilojoules. This value is a measure of the total energy expended over the course of your ride. This is roughly equivalent to dietary calories expended.

Which is correct. The amount of kJ of mechanical work done roughly = Calories metabolised. It's more like 1.1 times kJ of mechanical work = Cal metabolised (give or take 5% or so depending on GME).

 

[elapid]

So the answer to bladerunner's question is still no conversion required because energy expenditure = calories burned

The amount of kJ of mechanical work done roughly = Calories metabolised. It's more like 1.1 times kJ of mechanical work = Cal metabolised (give or take 5% or so depending on GME).

and not

If you want to convert that to Calories (kcal), then divide again by 4.18 (kJ per kcal).

 

[Alex Simmons/RST]

So the answer to bladerunner's question is still no conversion required because energy expenditure = calories burned

He would need to know his (typical*) GME in order to know the multiple for him. kJ of mechanical work to Cal metabolised might be a multiple of 1.0, but it might be also be a multiple of 1.2. That's quite a big difference.

* of course an individual's GME is also subject to acute variations due to a range of factors

 

[elapid]

But, bottom line, while it is still an estimate (as is every other bike computer), work done in kJ on a PT = calories burned. No need to divide kJ by 4.2 to calculate calories burned because this figure is then divided by 0.25 (or whichever GME figure Cycleops uses as an estimate for GME) (or multiplied by 4) and that is why kJ is roughly equivalent to calories burned on a PT. Yes, it is an estimate because it assumes a set GME and GME can vary widely depending on technique and experience. But still kJ = calories burned on a PT and this is because of the reasons stated both above and in my first post, which was based on information given to me by my coach and Cycleops.

 

[Alex Simmons/RST]

But, bottom line, while it is still an estimate (as is every other bike computer), work done in kJ on a PT = calories burned. No need to divide kJ by 4.2 to calculate calories burned because this figure is then divided by 0.25 (or whichever GME figure Cycleops uses as an estimate for GME) (or multiplied by 4) and that is why kJ is roughly equivalent to calories burned on a PT. Yes, it is an estimate because it assumes a set GME and GME can vary widely depending on technique and experience. But still kJ = calories burned on a PT and this is because of the reasons stated both above and in my first post, which was based on information given to me by my coach and Cycleops.

Once again, The Powertap makes no such assumption on GME. Neither do the makers of Powertap say it does.

The Powertap reports kJ of mechanical work done. That is all.

How you decide to convert to an estimate of calories metabolised is up to you.

 

[elapid]

Once again, The Powertap makes no such assumption on GME. Neither do the makers of Powertap say it does.

The Powertap reports kJ of mechanical work done. That is all.

How you decide to convert to an estimate of calories metabolised is up to you.

Cycleops (makers of PT) customer support do say that work done (kJ) approximates calories burned. They base this on the formulas stated in previous posts (divide kJ by 4.18 to convert to kcal and divide this by a GME of 0.25).

This is supported by the PT instruction manual which, as quoted by you, states:

The total work done over the course of the trip or interval is shown in kilojoules. This value is a measure of the total energy expended over the course of your ride. This is roughly equivalent to dietary calories expended.

So yes, you can calculate your GME and do the appropriate calculations to work out your exact calories burned. Or, alternatively, you could use the kJ as an approximation of calories burned, which is sufficient for most athletes. This approximation is far more accurate than most computers which base their calorie expenditure estimate on heart rate and body weight.

 

[Alex Simmons/RST]

Cycleops (makers of PT) customer support do say that work done (kJ) approximates calories burned. They base this on the formulas stated in previous posts (divide kJ by 4.18 to convert to kcal and divide this by a GME of 0.25).

That assumes a G(rear wheel)ME of 0.239, not 0.25.

Given typical drivetrain losses of ~ 2-3%, that's a bit high for most. I'd suggest a multiple of 1.1 would be closer to the average mark for a trained cyclist.

 

[bladerunner]

Thank you everyone for your input. I will just use/apply the PowerTap Energy "number" as the approximate number of Kcals burnt.
Cheers.