cuocciom said:

I'd like to add a bit to your question. I bought a PowerTap SL this winter. I can track work in kilojoules for each ride. I've read that kilojoules roughly equals kilocalories burned; if true, then I should look at my kilojoules and have a fair idea of what I've burned. On the other hand, I've read that there is a conversion of 4-5 calories per kilojoule. If true, then I'd be burning 4-6 thousand calories a day for 35-50 miles of riding. This seems ridiculous to me.

My problem, I believe, is that I don't know what kilocalories and kilojoules mean in non-scientific terms. Are there others out there who share my confusion? Can anyone explain this in terms that a non-physicist can understand?

Lets have a quick go...

Each Kcal is worth ~4.18 Kj, i.e., to arrive at energy expended (from PT/SRM data) in the usual 'food' metric (Kcal) you divide your power meter reading by 4.18. Lets say you rode for several hours and expended 2000 Kj, your Kcal expenditure was therefore, 479 Kcal.

HOWEVER, humans are ~ 20 - 25% efficient when cycling. Therefore, to arrive at the actual expended energy in Kcal you need to multiply that 479 by a factor of 4 to 5, i.e., 1916 to 2395 Kcal. Somewhere between these two figures is the likely amount of energy that you expended in Kcal.

If you knew how efficient you were you could more accurately calculate it, BUT, efficiency alters for the absolute power you ride at, the cadence you use, and environmental and topographical conditions. Unless you ride round with a portable gas analyser (to measure the air you breathe out) you won't know the exact value of your efficiency.

We can therefore, ballpark our energy expenditure in Kcal simply by saying that whatever your power meter said in Kj is the Kcal expenditure, i.e., in this case you expended ~ 2000 Kcal.

Cheers

Ric