How accurate are gym equipment calorie burn measures?

June 9, 2008 3:52 PM

How does my excersise bike calculate calories based on weight, and how accurate is it?

If I tell my exercise bike my weight, it tells me how many calories I've burnt. Mrs t42 does the same, with a lower weight, and gets a lower calorie count for the same time.

All good... except I'm not moving any more weight than she is. Sure, my legs may weigh a little more, but my bulk is stationary and the amount of effort required comes from the magnetic resistance of the machine.

So, is the calorie count in any way accurate? How do I test it without spending lots of money?

As, I guess, a secondary question - how accurate as the calorie counting heart rate watches? I understand that your calorie requirements can be estimated from your heart rate, but surely blood pressure, density and heart *size* must make a massive difference to the answers...?

posted by twine42 to health & fitness (14 comments total)

So, is the calorie count in any way accurate? How do I test it without spending lots of money?

It's a guess. If it's a good guess its calculated with decently researched equations. Google calories burned per hour and stuff like that for an idea of the wild variability of equations people are using for this kind of thing. Some running sites say I should burn 100 calories/mile at a moderate pace, but I've seen others that claim it's close to 300. That's a huge range, and if you're watching your weight it makes the guesses worthless.

You can test it extremely cheaply by counting every single calorie you take in during a day, over about a month. (That's daunting, but it can be fun and is a powerful dieting tool.) If you keep your weight the same that will tell you your metabolic rate while exercising (or your basal metabolic rate if you're not exercising). Then you can increase/decrease the amount you're working out over two months or so and compare that to the calories the machine says you've burned. If you're accurate with your calorie counts (and don't change your diet) you will gain or lose weight. If the machine says you've burned 24500 (7lbs) calories over that time but you've only lost 2lbs (7000 Calories) you know the machine's count isn't accurate. But remember, the quality of the data you put in determines the quality of the information you'll get out of this process. Calorie counts must be accurate.

It is a lot to do, but it's essentially free. I know, from using this technique that I need 13.5 Calories/lb of body weight per day to maintain my current weight. I also know that while running at a moderate pace (7.5-8min/mile) I burn 130-150 calories/mile. I've got daily weights and calories eaten daily logged for about a year and a half. I'm very confident that this is accurate for me.
posted by Science! at 4:20 PM on June 9, 2008 [2 favorites has favorites]

So, is the calorie count in any way accurate? How do I test it without spending lots of money?

The thing is, most machines include your BMR in the calorie count they provide you for using that machine. You are not burning, say, 10 calories a minute out of the pure exertion of using the machine, you are burning 4 calories a minute of exertion, and 6 calories/minute just by virtue of your size, gender, age, and muscle mass (to put it another way: sitting on your ass for ten minutes only uses 40 fewer calories than using the machine). Of course, the machines lump that all into "size", thus, you burn fewer calories using the machine than your smaller wife.
Science! is really spot-on about using, well, science to determine your actual caloric needs. If you have the patience to do it, it is really the way to go.
posted by ch1x0r at 4:38 PM on June 9, 2008

you burn fewer more calories using the machine than your smaller wife
posted by ch1x0r at 4:40 PM on June 9, 2008

Don't have a source to site, but I thought I remembered reading that machines can be off by about 10%. So, if you burn "400 calories", it might really be 360.
posted by toaster at 4:51 PM on June 9, 2008

Those machines probably off by a LOT more than 10%.

My machine at home reports that I burn twice as many calories as the machine I use at the gym, even though I'm clearly exerting myself at roughly the same amount on both machines.
posted by mikeand1 at 6:20 PM on June 9, 2008

Supposing your legs are 10 lbs heavier than your wife's, your pace is 150 strides per minute (75 per leg) for 30 minutes, and you lift each leg about a foot in any given stride, you'd expect to burn about 73 Calories more during your workout. (One dietary Calorie is 1000 calories). These numbers are perhaps all a bit high, but 50 or more Calories per half hour is reasonable just from the work of moving your body mass up against gravity repeatedly (your torso will also bob up and down a couple inches, your arms are moving, and your body isn't 100% efficient). On top of that, you have more blood that your heart has to pump a greater distance.
posted by dsword at 6:23 PM on June 9, 2008

check out www.caloriesperhour.com for an example of your calories vs. your wife's calories. Those machines are terribly inaccurate, especially if they don't take into account your weight, age and, heart rate. Also, not everyone falls into the heart rate zones prescribed on the gym machines (the old 220 minus your age thing is just a ballpark figure). So, if you're anything like me, the machine will have you redlining at "above HR" while in fact, you are cruising along at an easy effort.

In general, I have found heart rate monitors to be fairly accurate. More accurate than the gym machines, at least. They tend to give you more accurate heart rate zones as well, especially with the brands like Polar, where you can do a test to determine your training zones. The Polar has worked quite well for me as far as calorie estimates go (meaning, I can lose weight by creating a calorie deficit following the calories burned estimate from the monitor and my daily caloric needs).
posted by smalls at 7:38 PM on June 9, 2008

Previously.
posted by betterton at 7:58 PM on June 9, 2008

Science! Great idea, but it's trickier than that. People's N.E.A.T. scales up and down somewhat to deal with caloric surplus/deficit where NEAT = Non-exercise Activity Thermogenesis. The Mayo clinic is doing research on this among others.

I'm not sure if the caloric expenditure calculation just represents the exercise activity calories (calories burned by legs, abs, diaphragm, heart to crank the pedals) or also the additional caloric burn represented by the increase in basal metabolism over the next few hours after exercise. Your basal metabolism burns calories in proportion to time and lean body mass (muscle). Exercise temporarily increases your metabolic rate, and that affects the caloric intake of all your muscles, not just the ones you just worked out.
posted by BrotherCaine at 11:08 PM on June 9, 2008

Also, for a low impact activity, I suspect the more your body fat %, the less accurate the calculation.
posted by BrotherCaine at 11:11 PM on June 9, 2008

Guessfilter: I would expect that more muscle mass means the muscle burns more calories even doing the same work/watts. I might be wrong, but it wouldn't surprise me that there is an efficiency cost to using a cannon to shoot a gnat, in terms of using people of different muscle mass to do the same work.

It's a bit easier with a real bike - find the elevation of the top of a hill, time how long it takes you to cycle up it, weigh yourself and your bike, then you know exactly how many watts you put into the bike during the workout (you could add a few more for friction, but with a bike at low speed up a hill, friction is pretty minimal). Knowing the watts, find the conversion for metabolic efficiency and body weight, and if the results are way off from a similar workout on the exercise bike, you know something is up.
posted by -harlequin- at 11:30 PM on June 9, 2008

(watt-hours, I should say)
posted by -harlequin- at 11:31 PM on June 9, 2008

There is also some evidence that the metabolic system, being a biological system (biological systems being known for their buffers and other status quo-keeping systems), may not in fact have a purely mechanical approach to incoming and outgoing calories/nutrients.

Also, there are foods and parts of foods that we eat that we cannot digest or metabolise that still technically have calories (in the strict technical sense, calories are determined by burning something in an insulated water bath and finding out how many degrees the water goes up by after the burn is finished). By that measure, cellulose (plant starch) has calories, but not all cellulose is actually digestible.

It sounds from the various comments here and linked comments elsewhere that the situation is at least as complicated from the point of view of metabolism, physiology, exercise and biomechanics.

I think with respect to bodily calorie burn, you can get better measures tailored to your personal physiology/metabolism by hooking yourself up to expensive analytical machines, but beware that we still don't understand very well why it seems that even the most effective diets seem to allow folks to return to their previous weight within 5 years.
posted by kalessin at 4:19 AM on June 10, 2008

To elaborate on kalessin's final sentence, VO2 max testing may be able to precisely determine caloric burn rate at various heart rates. I'm not a kinesiologist though, so I don't know if extensive anaerobic activity is accurately incorporated into such models.
posted by BrotherCaine at 5:45 AM on June 10, 2008