Friday, August 26, 2011

Don't Rely on Treadmill Calibration!

I've already confessed to being both a geek and a data junkie, so this post may not come as a surprise to any of the (no) people reading this blog. Once I had my new Garmin in hand and started using it, I started to see interesting discrepancies between what the treadmill's display was telling me and what the Garmin was telling me.

Now, I would expect that if it were a problem with the Garmin or its footpod, I'd see the same or very similar discrepancies all the time. But that's not what I'm seeing.

A few days ago, I used the third treadmill from the end at my gym and noticed that 3mph seemed exceptionally slow. After a bit, I glanced at the Garmin and it said that I was really walking at 2.8mph. hmmm.

Today I used the fifth treadmill from the end to see what happened. The treadmill's stated speed agreed with the Garmin's reading so it was probably reasonably close. But the problem came in mileage....

I walked for 2 min at 2.5 mph (a warm-up and a cool-down minute)
Then I walked for 58 minutes at a 3.0 OR GREATER speed. I was watching my HR monitor and would go between 3.0 and 3.2, depending on what my HR was doing, mostly 3.1-3.2.

At the end of the hour total time, the treadmill told me I had walked a total of 2.75 miles.

Ummm.... that math does NOT work at all.

58/60 = .97 (the portion of an hour actually walked in full-on mode)
.97 * 3mph = 2.9 miles (minimum distance because I'd actually spent most of it at 3.1 to 3.2 mph).
1.0-.97 = .03 (the portion spent in warm-up/cool-down mode)
.03*2.5mph = .075 miles (distance walked in warm-up/cool-down)
2.9+.075 = 2.975 miles

2.975 is the MINIMUM I walked. This is a difference of almost a quarter mile from what the treadmill told me. The difference is greater if I take 3.1 mph as an average and do the same calculation.

Seriously - walking 2 minutes short of an hour at a 3mph pace should give me just shourt of 3 miles, no? Not 2.75.....

So, in light of all the variances, I'm going to stick with the Garmin's variances since they should be consistent over time. I think my best bet is to walk for an hour and just see how far I go, since I know I can walk 5K in an hour at a very doable pace.

So - this is my data geek warning to never take what the display on any exercise machine tells you as the truth. Calibration (or lack thereof) can play havoc with the stated values.

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