If you've ever used, for example, a stationary bike in a gym, you've noticed that all modern bikes have a screen that shows energy burn, typically in Calories. How does the bike know? I enter my age and mass before starting my bike work. The machine doesn't know my gender, my height, or my level of physical fitness. My heart rate is determined by an electronic sensor in the handle I hold while biking, but that's it for biometric data. So general population statistics must be used in the determination of energy burn. That's a lot faster than having to enter gobs of information before exercising, some of which we probably wouldn't want to enter anyway. And besides, how accurately do you

*really*need to know how many calories you burned? You certainly want the number to be reasonable. After all you don't want to spend an hour on a bike and be told at the end of that hour that you burned 6 Calories. The idea is to provide a reasonable estimate, which mostly serves to help me set goals.

*Tomorrow I'll burn five more Calories!*

*How much energy do 10 Calories represent? Well a "calorie" is an energy unit that scientists found useful while doing calorimetry (note the similarity?). The "thermochemical calorie" is defined to be exactly 4.184 joules. Other definitions of "calorie" involve how much energy is needed to increase water's temperature a certain amount at various starting temperatures. All those definitions give approximately, 1 cal = 4.2 J. The "kilocalorie" is what we think of as a nutritional calorie. Often called a "large calorie," the unit is written "Calorie" with a capital "C" to distinguish it from a normal calorie. Nutritional labels that state how many "Calories" are contained in food really mean "kilocalories." Here in England, I often see "kilojoules" or kJ on food packages. That means 1 kcal = 1 Cal = 4.2 kJ (good to two significant digits). My per minute burn goal of 10 Calories thus translates to about 42 kJ per minute.*

Let's suppose I bike for a full hour and I'm told that I've burned exactly 600 Calories, thus achieving my 10 Calories/minute goal. The computer determined the energy burn from average population statistics, as I've mentioned, and from simple physics. It's not hard to calculate how much work is done with each turn of the bike's wheel at a given resistance setting. To check the bike's accuracy in determining my energy burn, I prefer to work with power, which, in this case, is the rate at which energy is burned. Burning 10 Calories per minute turns out to be equivalent to an average power of 697.8 watts. Instead of working with all those digits, let's just round up and say that 10 Cal/min corresponds to 700 W. Is that a reasonable power output? Not for me! During last year's Tour de France, the black sticks you saw on bikes determined, among other things, a cyclist's mechanical power output. A few cyclists published their power data, and for some of the big climbs, power outputs were in the range of 300-400 W. Do I seriously double the power output of an elite cyclist at the gym each morning while I'm watching news on the screen in front of me? No!

So what to make of 700 W? Well the bike's computer is trying to tell me how much energy I

*burned*, not my mechanical energy output. Energy conversions in the body and with muscle actions, as with car engines, are not even close to being 100% efficient. Muscle efficiencies depend on which muscle groups are used. Some exercise equipment may even add the energy burned from normal metabolic processes that take place, even when we're not on a bike. In my Tour de France research, I've used an average efficiency of 20% to estimate how much energy a Tour de France cyclists burns for each stage. Suppose the bike I use in the morning skips my normal metabolic burn and uses 25% for its efficiency conversion calculation. That means my power output was just 175 W. Now that's more reasonable! At that power output, I'm about half a Tour de France cyclist on a steep climb. I could output more power, but I couldn't maintain that power output for an entire hour. That Tour de France cyclists can output 300-400 W for a half hour or more is testimony to just how fit they are.

Next time you're in the gym, note your energy burn on the machines that provide such information. Then calculate your average power. See how close you get to an elite athlete like a Tour de France cyclist.

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