# Gym question: How does a machine calculate calorie expenditure?

Anyone who’s been in a gym knows that many of the aerobic/cardiovascular-type machines, like the stationary bikes, treadmills, and elliptical crosstrainers have handles you can hold to measure your heartrate. Some of these machines ask for your age and weight and use that to determine your optimum heartrate for “fat burn” and “cardio” exercise. Some don’t ask for any information at all.

Okay, fine, I guess some physiologist or exercise scientist figured these ratios out through experimentation and research. If the machine tells me I’m exercising at a rate that will afford maximum benefits to my cardiovascular system, I’m not exactly prepared to argue with it.

But what exactly is with these calorie counters? Suppose I use the treadmill for thirty minutes and it tells me I’ve burned three hundred calories. Why is it that the next day I get on the elliptical crosstrainer and do (I think) the same amount of exercise, and it tells me I’ve burned six or seven hundred calories?

The bikes generally don’t ask for your weight at all. I suppose that’s because, since you’re sitting down and not really moving your weight, it doesn’t really matter how much you weigh. Is that correct?

I’d love to know the formulas they use to come up with these calorie consumption rates. I googled, but all the information seems to be in the form of charts with specific activities and durations. Anyone know how they figure these things out?

the purpose of asking for your age is to calculate your max heart rate which is 220 less your age. Percentages of that number are then used to calculate the heart rate you need to obtain your goal which will either be a cadio workout or a fat-burning workout. Your heart rate is used to determine where your body is getting energy from…fat, carbohydrates, muscle breakdown and muscle glycogen.

At your max heart rate and above, a very high % of your energy is coming from muscle breakdown and muscle glycogen…at about 80% MHR (max heart rate), you are using a high % from carbohydrates…at roughly 55% mhr, you are using a high % of fat for energy.

The actual calorie counter is usually just a calculator based on your weight.

treadmills - unless you have the incline up to at least 3%, you basically just skipping on the tread and not really moving your weight. The movement of your body is producing the work…kinda like how it would in an aerobics class. the eliptical trainer is cheat free so to speak so therefore you work harder, it is impactless so you feel like you are doing less work. You are right on the bike…you are not moving your body weight.

I remember some of the older stairsteppres where they had the two foot pedals attached with a chain, so as one foot went up, other went down…basically a teeter-todder…people actually thought they were working out.

Personally, I’d calculate your BMI and then use the METs on the machine to calculate your actual calorie burn…this will probably be more accurate. Imagine averaging the gas milages of 15 different types of cars and then using that to calculate the MPG for all of them…that’s basically what those calculators do.

Hmm. I think I fall into the category of people for whom the BMI calculators don’t work. It gives me a BMI of around 32 - “obese”. I’m definitely far from obese. Is there another piece of data I could factor in there?

Does this mean that if you’re working out at your maximum heartrate, your body actually starts eating its own muscles? :eek:

yes…but even top athletes can’t maintain that for a long periods of time…when mine gets to the low 170s, I’m sucking air big time.

This is oversimplified. How much fat and carbs you burn also has a lot to do with how long you exercise. If you want to lose body fat, you may need to do longer periods of cardio since the first bit (half an hour or so) the body would preferentially burn carbs, even with a lower heart rate. If you want to burn fat, you need to burn calories, and this is more easily done at higher intensities. Heart rates depend a lot on the person’s physical fitness and is not a good way to measure calories burned.

These machines also frequently display your Power production (in Watts)–that is, if you were using these machines to generate electrical power, that’s about how much you would get–it’s an actual measurement based on how much work you’re doing on the machine. They then extrapolate to get your Caloric burn rate by making a guess, based on your entered information, at how efficient you are at converting stored energy (i.e., the food you ate) into work (how much you move that treadmill). IIRC, humans acheive an efficiency on the order of 25%.

So what factors must one consider when determining the rate at which one burns calories while doing a certain activity for a certain amount of time? Does body fat percentage figure into it?