Cycling Watts/Calorie calculation

[RobM77]

Just an idle musing during a boring afternoon at work. I think I might have fried my brain at the gym at lunchtime, so would appreciate where I'm failing in this calculation. The cycling machine in the gym claimed today that I was using 12kcals a minute (720kcals an hour) when I'm pushing at about 80% on the bike for five minutes during circuit training. This is confirmed by an online search, because apparently an hour of hard cycling burns about 500kcals, which sounds about right if I backed off enough to last for an hour but pushed as hard as I could.

However, according to an online calculator, 12kcals is 50,000 joules, so that's 833 J/s or 833 Watts. However, according to many online sources, even a top amateur cyclist won't put out more than about 350 Watts. Even the figure of 500kcals an hour that I found is over 500 Watts, and that was for an average guy cycling hard for an hour.

Where am I going wrong?

[jimbobsimmonds]

Assuming muscles are 100% efficient would be the mistake (I guess).

[RobM77]

jimbobsimmonds said:

Assuming muscles are 100% efficient would be the mistake (I guess).

ah... the '350 watts' figure is at the crank/foot. That explains it. thanks :-)

[Fatman2]

I don't think the estimatates are particularly accurate on gym bikes (even Keiser bikes are hugely variable) so the readings need to be taken with a big pinch of salt.

Part of that measurement also includes your BMR (approx 100kcals) so if it were accurate then you'd need to subtract about 100 from that value.

A pro cyclist like Lance will put out about 400watts (average over a 100 mile stage) but this will be from a power gauge so won't be including BMR.

[NoelWatson]

mcelliott is doing over 1200cals per hour - how many watts does that equate to?

http://www.pistonheads.co.uk/gassing/topic.asp?h=0...

[johnxjsc1985]

I think if I remember 400K/cals an hour requires a pretty hard effort probably somewhere 20mph+.

[NoelWatson]

johnxjsc1985 said:

I think if I remember 400K/cals an hour requires a pretty hard effort probably somewhere 20mph+.

I think mcelliot is doing that. Eddy Merckx also did when he set the hour record.

[RobM77]

johnxjsc1985 said:

I think if I remember 400K/cals an hour requires a pretty hard effort probably somewhere 20mph+.

Averaging 20mph+ is quite common for the average keen road cyclist. Bear in mind my OP was referring to a 4 minute session in a circuit training set. I couldn't hold that intensity for longer than about ten minutes

[mattikake]

BMR is supposed to calculate cycling at 8.5 kcals per min = 510 p/h. Just done one today for someone. But that's just BMR. It's very general and doesn't say what sort of speed this is, if there is an incline, wind speed and direction, weather, tyre pressures, bike type etc. etc.

I don't think there will ever be a method of accurately measuring watts/kcals/Kj for any sort of exercise short of attaching electrodes to every single muscle fibre... and that would be just for a gym bike.

You can be pretty sure exercise machines will always err on the side of generosity though!

[johnxjsc1985]

NoelWatson said:

I think mcelliot is doing that. Eddy Merckx also did when he set the hour record.

I think he did 400 watts per hour.

[NoelWatson]

johnxjsc1985 said:

NoelWatson said:

I think mcelliot is doing that. Eddy Merckx also did when he set the hour record.

I think he did 400 watts per hour.

I think ~1300 calories per hour works out at more than that.

[johnxjsc1985]

Boardmans hour record was produce at 442W for the hour.Tony Romminger produced 460 watts which is the most produced for one hours(or it was then).

[NoelWatson]

johnxjsc1985 said:

Boardmans hour record was produce at 442W for the hour.Tony Romminger produced 460 watts which is the most produced for one hours(or it was then).

I think mcelliot is close to that @1300 cals an hour

[johnxjsc1985]

cal per hour hasnt got anything to do with power output a big guy will "burn" more calories to produce the same power.Hence when going up the mountains the little guys can churnout 400watts all day where the bigger guys will expend far more energy. You can measure power output with SRM cranks its difficult to measure K/cals for a certain power output.

[NoelWatson]

johnxjsc1985 said:

cal per hour hasnt got anything to do with power output a big guy will "burn" more calories to produce the same power.Hence when going up the mountains the little guys can churnout 400watts all day where the bigger guys will expend far more energy. You can measure power output with SRM cranks its difficult to measure K/cals for a certain power output.

I'm not sure why there is not a relationship between power output and calories burnt. Surely they are linked by muscle effiency (~25%)?

[johnxjsc1985]

Power output is measurable by individual, I dont think there is an accurate measurement of calories burnt. Estimate yes but is there an accurate measuring device?. Some people burn calories more efficiently than others hence some people are good at long distance and some at short distance.
I used to be able to handle races of 2 hours above that I struggled. Others would become stronger the longer the race if we both started of with similar amounts of fuel/food on board I must have used mine quicker/less efficiiently than the guy who can do 12 or even 24 hour timetrials.
I guess the fiter you are the more efficiently you will burn fuel also.

[johnxjsc1985]

I think some are getting K/cals and Watts mixed up. Chris Boardman managed way over 400 watts for an hour at Manchester .400 watts is in the region of 30mph+
Calories as I understand is estimated from the watts produced via a formula it isn't measured per individual as watts can be.
Cycling is all about power to weight and calories dont really come into it as a measure of progress or speed.

[johnxjsc1985]

Its about 5 years since I last raced but if you try www.analyticalcycling.com you can get lots more info on power outputs etc.

[NoelWatson]

swerni said:

In an interval session he was holding 400 watts over a 4 minute period and over the 30 second sprints he was starting at 500 watts and that was dropping off rapidly over the sprints. ( i was nowhere near these numbers, not even bloody close)

I would estimate I get around those numbers on the Concept 2 for the four minute and slightly more for 30 second sprints - but I'm probably a lot heavier.

http://concept2.co.uk/training/guide/watts_convers...

I drops to around 250w for half a hour.

Calorie wise (for 30 mins) I get a very high number using formula below - doesn't seem right.

http://concept2.co.uk/training/bmr

[johnxjsc1985]

I am getting about 900k/cals reading for a 40 min session on the Concept at around 2min pace which seems high at 1350p/h.