calories vs watts on concept 2 monitor

[danielshoe]

I exercised for 30 minutes on a Concept2 rower and it told me I generated an average of 133.1 watts and used 379 calories. But 133.1 watts for 30 minutes is 239580 joules, which given the generally accepted efficiency of the human body to convert food to mechanical work of 25% works out to only 229 calories.

that is (133.1 watts * 1800 seconds)/( 0.25 efficiency * 4814 joules/ calorie) = 229 calories, not the 379 the machine reported.

Is this because the Concept 2 thinks people are only 15% efficient in this regard, or perhaps the 379 calories are correct and the 133 watts was really higher?

Any comments?

-Dan

[Nosmo]

Ignore Calories. It is a relatively useless reading.
The C2 assumed a (I think a 170 lbs person rowing at 30 spm), and did actual measurements and a least squares fit to come up with the formula. The further you are from the size, rating, power and technique that C2 used to generate the formula the less accurate it will be.

25% is actually rather high. 20% would be more reasonable for most exercises. Also the C2 does not measure all the work done by a rower, only the work put into the fly wheel. The efficiency calculation will change depending on how fast you are rowing. (The one time I did the calculation based on my workout I got 18%).

[jamesg]

As well as converting Watts to kCal/h directly, at efficiency 25% as you say, the counter adds on 300 kCal/h for staying alive, moving up and down etc, i.e. all the work done other than that going into the handle. This is the 150 difference you saw over ½h.

[iain]

I exercised for 30 minutes on a Concept2 rower and it told me I generated an average of 133.1 watts and used 379 calories.

Apologies fro restating, but basically:

James has explained that the monitor assumes 25% efficiency on powering the handle and adds 300kCal/Hr for other energy used. The main difference is due to the sloppy use of calories when connected to exercise. A dieticians calories are the kilo calories produced when food is burned completely. In contrast, the erg measures the average mechanical energy dissipated by the flywheel. As you rightly point out there are many inefficiencies that the "conversion" attempts to approximate.

I would say that you should not expect the cals used to be accurate, but it is a reasonable guide. Better approximations are available, but they are time consuming. Given the imprecision of calorie counting, it atleast gives a reasonable indication. However you might like to mentally adjust if you are a long way from the 75kg standard or are exercising at very low intensity. I came across someone doing 500cal/hr who was very proud of her "additional" 250 cals used in her 30' row. At her 3:00 split the approximations make this value only useful for qualitatively comparing similar rows.

Regards

Iain