Monday, June 21, 2010

Calories Burned on a Bike Ride

Before I get started, I want to mention that I am not worried about how many calories I'm burning during exercise because I am not trying to lose weight. I am a scientist though, so I keep track of numbers and when I see a discrepancy, I want to know why. So here's the situation:

  • 22-minute bike ride in the neighborhood
  • I start first with a big hill down to a stream crossing. Then the rest of the ride is generally a slow increase in elevation all the way back to my starting point.
  • Average speed = 14.5 mph with a maximum 32.4 mph
  • Distance = 5.3 miles
  • Average heart rate = 156 bpm with a maximum 171 bpm
  • According to onboard bicycle speedometer programmed for my weight, I burned 166 calories
  • According to a fitness website in which I entered weight, gender, and average speed, I burned 288 calories.
  • According to my heart rate monitor (Timex T5G981 watch with chest strap--seems to be discontinued) programmed for my weight and age, I burned 388 calories.

So, which is to believed? Well, the speedometer on my bicycle doesn't account for the change in elevation during my ride, nor wind resistance. I would certainly be working harder when going uphill or against the wind. Thus, the speedometer is likely to underestimate calories burned.

The fitness website is imprecise because I couldn't enter my actual average speed; instead, I had to choose the 14.0 - 15.5 mph range. Plus, the same problem noted above regarding elevation change and wind resistance is unaccounted for in the website calculation.

The heart rate monitor's estimate seems most accurate to me because it is a direct measurement of my physiological condition; it knows how hard my body is working, but of course, it doesn't know what kind of exercise I am doing. So this is problematic as well, because hypothetically I could be on medication that raises my heart rate to 156 bpm and I wouldn't have to be doing any exercise. I have a triathlon friend who told me not to believe the heart rate monitor's estimate for that reason. She said the heart rate is a reflection of perceived work and not actual work. For instance, if you feel bad or are upset or are tired, your heart might be 156 bpm during an exercise than on a normal day would only raise your heart rate to 140 bpm. She is right. But I'm not sure that means there is a problem. After all, when one is feeling ill, your body is working hard to fight off infection. That takes energy, and you are burning extra calories. When one is upset, your brain is using extra calories to think about the situation. When you are tired, your heart is pushing blood through tissues that are fatigued, and...well I can't argue against this one. You would be burning fewer calories in this case because the tissues will be less efficient.

While none of these estimates of calories burned is perfect, I think the heart rate monitor is still the best of the three. As long as I am healthy and operating at average capacity, heart rate is able to take into account the physiological work I am doing, whereas the other two estimates have no way to integrate the difficulty of the course.

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