A disappointing day of cycling with the bodybugg

Up until yesterday, I’ve been generally delighted with the bodybugg.  It basically does what it is advertised to do, and provides very helpful feedback on my diet and exercise (i.e. I can see if it’s making a difference).  I really haven’t even started to use it for the fun experimentation I have in mind, and I’m looking forward to that.

Yesterday, though, the bodybugg let me down.  Here’s the scenario.Daniel and I had been planning for a while to do what he calls “the big Hard Guy loop.”  As I mentioned in another post, Hard Guy is an advanced mountain biking trail that has long been held up to me as one of the most challenging partial-day rides around here.  A couple of weeks ago we did the Hard Guy trail itself, but were waiting for a Sunday when we would have abundant time to do the big loop.

The big loop starts with three miles of constant climbing on the road to get to the beginning of the dirt trail.  It then proceeds as we did it before, with a little over six miles of almost constant uphill on a dirt trail (sometimes single-track, sometimes old Jeep trail).  When we did it before, we turned around at that point, and rode back down to the pavement where we’d parked.

Instead of stopping there, though, the big loop climbs a dirt road for another 500 feet of altitude in about a half-mile of distance.  Yeah, that hurt.  We stopped to let the nausea pass and had a quick snack, and then rode down the other side of the summit for a while on road, hit a couple of single-track trails with downs (Scott’s) and ups (Corrals), climbed back to the road, went down a short trail, and looped back to our original starting point on one of my favorite trails (Crestline-Kestral).

All said and done, it comes to about 22 miles (as opposed to the 12 or so for just the Hard Guy trail).  I had extra motivation, too, because Daniel bribed me with a rain shell if I did the whole loop.  I felt like I did reasonably well, not taking any breaks (even though I was so tired) except for the one at the summit, so I deserved my prize.

When we finished up, Daniel looked at the Polar heart-rate monitor he was wearing and read off the stats: 4 hours, 25 minutes of riding; 2550 calories; average heart rate (which I can’t remember, but know was annoyingly low because he’s annoyingly fit).  I was very tired and looking forward to getting home and syncing my bodybugg to see how many calories I’d burned.  Considering that the much shorter Hard Guy ride a couple of weeks ago burned 954 calories, I was also looking forward to a good lunch.

Imagine my surprise (read: freak out!) when I downloaded my data and was told I’d burned just 1212 calories!  In four and a half hours of killer hard work!  (Okay, some of that was downhill… but only about an hour of the total.)  Arrggg!

I felt terrible.  I’d put in this ton of effort, and apparently it was really minimal in the end.  I was ready to go all out depressive.  Fortunately, Daniel popped in to see the results and immediately protested their accuracy.  I argued feebly for the accuracy for a little while, and he asked how the results are calculated.  I pulled up bodybugg’s “The Science Behind bodybugg” page and had him read it.  There were a few things that bugged his engineer-brain:

•  The “Heat Flux” metric that’s a core part of the way the bodybugg calculates work.  One thing I didn’t mention above was that the temperature was mostly in the 90s (Fahrenheit) for our ride yesterday (as opposed to 60s and 70s a couple of weeks ago).  Daniel thinks that maybe the bodybugg wasn’t accurately recording this metric since the air temperature was about the same as the heat I was dissipating.
• The bodybugg doesn’t seem to track heart rate, which is a pretty accurate indicator of cardio exertion.
• The accelerometer seems like it would be less helpful on a bike, particularly on uphills where the forward progress is extremely slow, but very steady in both forward speed and on the various axises (axes?).  I expect that the accelerometer actually has more relevance going downhill, when there’s a lot of motion in all directions (especially on my hard-tail bike!), but the effort level is just a fraction of the climb.

Honestly, we have no idea why the numbers were so low.  I’m going to ask my bodybugg trainer about it on our call tomorrow.  But Daniel (together with the comparison data from the shorter ride) has persuaded me that the numbers really are low.  Even bodybugg’s calorie calculator suggests I should have burned at least another thousand calories.

At this point, my biggest question is: if it doesn’t give me accurate data for my chosen sport (cycling in various temperatures!), should I try to return it and get my money back (and buy a sweet Polar HRM), or keep it for rest days?  What do you think?