I spent a few more minutes on the elliptical at Ponds Forge this morning compared to my normal workout. When I was finished, I snapped a photo of the screen, shown below.
I have written about numbers seen on gym equipment before (click here), but I thought I would revisit the topic. The power reading was dropping precipitously after I stepped off the pedals. I had burned 684 Calories. Note that 1 "big C" calorie is the same as 1 kilocalorie, which is 1000 "little c" calories. Food calories are always in terms of "big C" calories.
To find average power, I need only divide energy output by exercise time. I will find the time in seconds first.
\[T=(80\,\mbox{min})\,\left(\frac{60\,\mbox{s}}{1\,\mbox{min}}\right)+4\,\mbox{s}=4804\,\mbox{s}\,.\]
Now find the energy output in joules.
\[E=(684\,\mbox{Calories})\,\left(\frac{4184\,\mbox{J}}{1\,\mbox{Calorie}}\right)=2,861,856\,\mbox{J}\,.\]
Sorry about all the digits! I will truncate soon. Now get my power output.
\[P=\frac{E}{T}=\frac{2,861,856\,\mbox{J}}{4804\,\mbox{s}}\simeq 596\,\mbox{W}\,.\]
That looks like an impressive power output, right? After I stopped the machine, I saw the screen below.
Ignoring the extra second on the clock, that page told me that my average power output was 98 W. Why such a difference in powers?
The answer lies in the thermodynamic inefficiencies associated with energy conversions in the body. The laws of thermodynamics tell us not only that we cannot get more energy out than what we put in, we cannot even break even. The glorious second law of thermodynamics tells us, in a hand-wavy way, that there is always waste energy in the workings of a motor. My body's efficiency may be found as follows.
\[\eta = \frac{P_{output}}{P_{internal}} \cdot 100\% \simeq \left(\frac{98\,\mbox{W}}{596\,\mbox{W}}\right)\,(100\%) \simeq 16.5\%\,.\]
I have added subscripts to the powers. My output power was not quite that of a 100-W lightbulb. If you have never tried to keep a 100-W lightbulb on while biking on a stationary bike, you should try it one day. It is not as easy as it might seem! But for me to output power at the rate of 100 W, I need to consume food energy in my body at a rate nearly six times what I am able to output. The waste energy is why I feel hot while working out. I have estimated Tour de France cyclists' body efficiency to be 20% in my past work. That is certainly in the ballpark with what I found above.
The MOVEs number is a bit less well defined. Technogym, who made the elliptical I used, defines MOVEs as "... the ultimate unit of measurement to objectively assess your lifestyle based on the movement and exercise you do and regardless of your gender, age or level." Technogym gives some examples, such as 50 MOVEs for taking five flights of stairs, 200 MOVEs for a half-hour walk, and 400 MOVEs for a half-hour run. Technogym further defines Movergy as "... an index of how active your lifestyle is and represents the average of your daily MOVEs over a 14-day period." If your Movergy is below 500, you are sedentary. If your Movergy is above 750, you are moderately active. And if your Movergy is above 1000, you are considered active. I am thus happy with my 1629 MOVEs!
Do take numbers on gym machines with a grain of salt. The machine I used had no idea of my gender, age, height, weight, level of fitness, and so on. I suspect that a human average efficiency of one-sixth is used to compute energy burned.
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