View Full Version : How does Keiser convert watts to kcals?

sharimiranda

03-11-2012, 09:00 PM

I have a member who's asking this question:

"The Keiser M3 computer must have a formula that it uses to convert watts to kcals. I'm curious. What's the formula? I went to the Keiser site, but I couldn't find any documentation."

Can anyone out there help? Thanks!

Shari

CycleGuy

03-12-2012, 12:54 AM

I have a member who's asking this question:

"The Keiser M3 computer must have a formula that it uses to convert watts to kcals. I'm curious. What's the formula? I went to the Keiser site, but I couldn't find any documentation."

Can anyone out there help? Thanks!

Shari

Somewhere on the forum, I think that someone mentioned the computer does a look up on stored data. If resistance is x, cadence is y, then power is z. The data table was created on some tests that Keiser did and assumes that the bike is calibrated the same as the test equipment. YMMV - or in this case - YPMV (your power may vary).

InnerDrive

03-12-2012, 05:06 AM

I'm not sure what method Keiser bikes use to estimate power, but my guess would be that calorie expenditure estimates are based on the calculated power output (which may not be all that accurate...) - watts, joules/kilojoules, and calories/kilocalories are all measurements of energy expenditure.

1 watt = 1 joule/second

1 calorie = 4.182 joules

The "calories" we talk about relating to dietary intake & energy expended during exercise are actually kilocalories (1,000 calories).

I don't know if the calorie estimates on the Keiser computer account for ineffiencies of the body to convert food to energy, etc.... My guess is they don't....

Here's a pretty good explanation: http://brisbanebike.blogspot.com/2011/07/watts-and-cycling.html

Vivienne

03-12-2012, 05:10 AM

1 Watt=1joule/sec. So total energy output over a course of time= wattsxtime (in seconds)

There are about 4.19 joules/cal (or kjoules/kcal) but, assuming that the human body is only about 25% efficient at converting chemical (food) energy into mechanical energy (work), you make the assumption of a 1:1 ratio or thereabouts (i.e. it takes about 4 kcals to do 1 kjoule of "work".

The inaccuracy comes from the fact that a Keiser Watt may not actually be a *James* Watt of 1 joule/sec as measured by a strain gauge and the assumption of biomechanical efficiency. It's a good enough proxy to compare work done from one workout to the next on the same bike but isn't an accurate representation of numbers of chocolate chip cookies burned (which is what class members are usually asking about......not high school physics)

Todd knows this one for sure, but he's prolly still asleep .....he can give a better explanation in a few hours.

Vivienne

Vivienne

03-12-2012, 05:14 AM

I'm not sure what method Keiser bikes use to estimate power, but my guess would be that calorie expenditure estimates are based on the calculated power output (which may not be all that accurate...) - watts, joules/kilojoules, and calories/kilocalories are all measurements of energy expenditure.

1 watt = 1 joule/second

1 calorie = 4.182 joules

The "calories" we talk about relating to dietary intake & energy expended during exercise are actually kilocalories (1,000 calories).

I don't know if the calorie estimates on the Keiser computer account for ineffiencies of the body to convert food to energy, etc.... My guess is they don't....

Here's a pretty good explanation: http://brisbanebike.blogspot.com/2011/07/watts-and-cycling.html

There'd have to be some compensation somewhere otherwise the Cal readout would be 4 times too big and that'd look really obvious. I knew there was an exact formula for calcs somewhere. Thanks for this link.

Vivienne

Todd S

03-12-2012, 09:59 AM

This has been discussed in detail here before if someone has time to do a search. The one calorie = 4.182 joules and the 20 - 25 percent gross efficiency of the body when converting chemical to mechanical energy makes the whole 'rule of thumb' 1 kJ of work = 1 Calorie (capital C) relationship work out quite nicely. (And it's more than accurate enough to serve the purposes of someone exercising.)

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