We all know that I’ve got a bit of a toy habit, or at least we do now, so it is a little disingenuous for me to describe a PowerTap as my “Christmas Gift” to me, but that’s approximately what the rationalization was. What’s a PowerTap? We’ll get to that in a minute. First, we have the Song of the Blog:
Like the crack of the whip, I snap attack,
Front to back, in this thing called rap.
Dig it like a shovel rhyme devil,
On a heavenly level,
Bang the bass, turn up the treble.
Radical mind, day and night, all the time
Seven to fourteen wise divine
Maniac brainiac winning the game
I’m the lyrical Jesse James
Back in December, Bob and Mom gave me a measurement package from Third Coast Training. While that was great to give me baseline numbers for how my metabolism worked in conjunction with my ability to pedal, I also wanted to have better data as I was riding to understand how I was progressing and what kind of calorie expenditure I was producing. For that, I needed a way to measure my power output, and that’s where the PowerTap comes into play. Beware, extreme cycling geekery begins here…
For those of you who have managed to put freshman physics behind you (did I mention that one of my riding buddies is Dr. Duck’s son-in-law?), here’s a quick refresher:
When one pedals, the circular motion of the crank arms is turned into linear motion of the chain, which is then converted into circular motion by the rear (drive) wheel. As a result, force is applied in a constrained, pivoting plane, aka torque:
Once you get back from running away from your computer, screaming, we can get back to the narrative.
So, why all the technical stuff, and by “technical stuff”, I mean physics I’m embarrassed to admit to having had to do a bit of re-reading to refresh my understanding? Well, it turns out that the body is actually a machine! Yep, 100% full-blooded Cylon, that’s me. I guess I just didn’t know it at the time!
It turns out that the human body is somewhere between 20% – 25% efficient in converting stored energy into physical work. In other words, since one calorie is 4.184 joules, and one joule is the base unit of work, you can approximate how many calories you burned doing exercise by looking at your total work output and changing the unit of measure. Don’t forget, though, that metabolically we deal in “C”alories (aka kilocalories), so you are really comparing kilojoules (kJ) to Calories. So, if you want to know how many calories you really expended on a treadmill, see if the machine will tell you your total work output in kJ or Watts (Watt is the measure of Power, which is Work over Time, but it gets confused because the letter “W” often represents total Work in classical mechanics problems). If it tells you Calories, you don’t necessarily know what conversion process the machine is using to get to that number. Here’s one way to tell: if your Calorie expenditure seems sensitive to your weight, then chances are that the machine is computing Calories somewhat or completely independently of its power sensor. This is called foreshadowing.
So, back to this PowerTap thing. The PowerTap is essentially a torque sensor (strain gauge) built into the rear hub that transmits data wirelessly to my Garmin cycling computer. So, whenever I pedal, it sends a signal to my computer with how hard I’m pedaling for how long. The computer then aggregates the data for viewing and later analysis (I love the download capabilities of my Garmin). This then leads to some interesting possible scenarios between what the sensor says versus what some of the embedded physiological models in the computer say about how much work I’m doing at any given time.
Tri-County Hill Hopper – 2/21/2010
Back in February, I did a ride that began in Round Top, about 20 miles outside of Brenham, and that covered a bit over 60 miles of fairly rolling terrain. According to my Garmin, the 67.74 miles covered 3,165 feet of climbing and caused me to burn 6,338 Calories.
However, when I pulled the workout file into a software package that is dedicated to analyzing files from power meters, it said that I had “only” expended 3,557kJ, or approximately 3,600 Calories. Why the discrepancy? Simple, really. The calorie expenditure in the Garmin has embedded in it an assumption that all of the motion on the bike is caused by the rider (true) constantly putting power to the pedals (definitely not true — thank whatever deity you want that for very uphill there’s a down!). So, we go from “crunch all you want” to “good ride, but let’s stick to only one dessert.” Poo.
