I was at the gym this morning, running on the elliptical trainer as is my usual habit, and I found myself pondering the little display that tells me how many calories I’ve burned so far in my workout. The same display tells me how many calories I’m burning per hour.
My question is, how is this calculated? It’s obviously a rough estimate at best, and computed using my speed and my weight (which I have to type in when the workout starts), but on what basis? The same goes for those exercise guides that say you burn x calories/hour sitting still, y calories per hour jogging, and z calories per hour doing yoga or whatever. How did we arrive at these numbers, and is there any reason to think they’re at all accurate?
I know there’s this thing they can do where they put someone on a treadmill and measure his oxygen consumption through something like a diver’s mask. I suppose if you repeated this with a variety of subjects and a variety of speeds and inclines and so on, you could start to come up with some figures.
You’re not breathing oxygen in and out for the fun of it; you’re using it to oxidise glucose (more or less) and that’s not a complicated equation to balance. So if you know how much O[sub]2[/sub] someone is using per minute, you can get from that to an amount of glucose being burned, and from that to calories is just a short hop given that glucose yields about 100 kilocalories per ounce. Then just take the figures and start interpolating and extrapolating, and off you go.
Work (foot pounds) and Calories are interchangeable units.
The machine probably does some calculation of how much work you’re doing, and converts to kcals. It probably guesses how much your legs weigh, based on total weight, and how much you move the rest of your body. Then again, as Malacandra suggests, someone may have measured oxygen consumption and gone from there.
Elliptical trainers work out how much energy you are expending by measuring the power output of the trainer. Basically, the trainer drives a generator. The load is dumped via resistors - the higher the load, the more the resistance in the generator (and thus on you). By measuring the power output of the generator, you know exactly how much effort the exercisee is using at the time. Stationary Bikes do the same thing. Some machines ask your weight to factor in the calculations.
Average power usage during activity or resting for a person can be calculated in many ways. Oxygen consumption is a start. Sticking someone in a roomsized calorimeter and measuring heat change in the room over a period of time is another. Much of this research has already been done in the past, and tables compiled that scientists use.
I have often wondered about the accuracy of these calories counters myself. I notice that some machines are much more generous than others. For example I can fairly comfortably burn 500 calories on an elliptical trainer in 45 minutes or so. On a treadmill I would be reduced to a quivering mass before I was able to burn the same 500 calories.
I think most of these machines include the base rate of calorie burn. That means the calories you would burn just being stationary. If one is looking to lose weight, you are probably more interested in how many additional calories you have used.
I don’t think that’s right, Mike, unless I’m not understanding you. All the treadmill I’ve ever been on (that allows you to punch in your weight) will give you the number of calories burned based upon how much exercise you do, and it’s not a constant number…
nevermind, I see what you’re saying. Yes, I think they do INCLUDE the base rate of calorie burn. But that’s the number most people want to know. I want to know How many calories I just burned on a treadmill. I’ve never thought "how many MORE calories did I burn on a treadmill as opposed to being completely stationary. The amount is probably negligible anyway. The most common formula that I’ve found for calculating resting burn rate of calories is weight X 11 for men, and weight X 10 for women = daily calories burned.
Hmm, total number of calories you burn on the treadmill can be added by calculating your BMR, dividing that number by 24 to find the hourly calories burned just sitting there. Add what you burned on your treadmill. Say I walk for 1 hour, in that hour I walk 4 miles. Say I burn 450 calories on the treadmill in that hour. My BMR is around 1800 calories a day, so 75 calories per hour.
I’ve had exactly the same experience Mm2. 40 minutes on the cross trainer allegedly burns off 550 calories or so; I do nothing more than tidal breathing. Running at 10kph for 10 minutes burns off about 160 calories, a roughly comparable rate of calorie consumption, but the running is a lot harder work!
You’re double-counting your BMR here. The number of calories burned in an hour of exercise is equal to the number of calories you would’ve burned doing nothing plus the additional calories that you burned by exercising. The treadmill is giving you an estimate of the sum of those two amounts, so you have to subtract off the first value before adding it to your BMR.
