Ok, I get the idea of measuring caloric intake using a bomb calorimeter. So, how does my treadmill know how many calories I’ve used up?
Related question: How would they measure the calories expended from giving blood or ejaculating? Surely calories go into the production and replacement of those cells.
I believe that accurate measurements of calorie expenditure requires collecting the air you exhale and analysing how much CO[sub]2[/sub] you produce. By collecting data on a large number of people and comparing to heart rate or some other reasonably simple-to-measure variable, it is possible to come up with indirect measurements with a fair degree of accuracy for most people. People with a particularly unusual physiology may be less well served by these gadgets.
Ok, that makes sense… so how would one measure calories used for replacing cells?
To add to the above, your treadmill uses a much less sophisticated method to guesstimate your caloric usage. It takes various parameters you enter, such as your weight and age. It then monitors the amount of energy to transmit to the treadmill during your excercise routine, and uses this to calculate your total caloric usage. The accuracy will generally be fairly poor, but close enough for reference.
I have to disagree with the second part of this. As you become more aerobically fit, the caloric expenditure necessary to perform a given feat of endurance decreases. The treadmill’s measurements have no means to track this.
In addition to ultrafilter’s point, it’s my opinion based on over 20 years of calorie counting and graphing loss/gain against fairly precisely measured caloric intake over the course of weeks/months/ years that the commonly assumed ranges for active people of 12-14 calories burned daily per lb of maintained body weight used in many assumptions about basal metabolic rate(s) are grossly incorrect for a substantial number of people, and that many active people (especially 40 years & older) can easily maintain a given weight set point on 9-10 calories per lb of body weight.
I think a lot of the assumptions we (and doctors) make about caloric needs across time are incorrect and treadmill measurements really don’t really get at real world BMR requirements across time in the real world.
From what I’ve read, such as here, http://www.active.com/story.cfm?story_id=12618 , the equations used are probably accurate to within 85%-90%. If the machine uses a heart rate monitor, the equations are supposed to be more accurate. I always think that it should be relatively easy to measure the calories expended on something that is not machine driven, say an exercycle, as opposed to something machine driven, say a treadmill. But then. the bike can only measure the energy you deliver, not the energy you expend.