I own a Garmin Edge 705 GPS unit for my bike and it counts calories for me as I ride.
I wonder how accurate it is.
I’ve given it my weight and the weight of my bike and I understand that if I pedal up a hill of a certain height then I’ve done a certain amount of work and necessarily expended a certain amount of energy, but what about rolling and wind resistance?
Wind resistance is not linear with speed, of course, and rolling resistance is dependent on the tire… and what about drive train friction? Is Garmin smart enough to factor all of this in?
How accurate is my calorie meter?
Thanks.
Like many such things, it uses a model of calorie expenditure that may or may not reflect your own personal metabolism and equipment.
According to the Garmin forums, earlier Garmin models use a speed/distance/time calculation for calorie expenditure (no heartrate info). This calculation can be anywhere from 10%-60% over. These calculations may have been addressed in recent firmware updates. More recent models (405XL is mentioned) use FirstBeat technology from Polar to give a much more accurate estimation of calorie expenditure. Licensing costs seem to be preventing the technology rolling back into older devices.
Some people recommend telling your garmin that you are about 15%-20% lighter than you really are to get a more conservative calorie expenditure figure, but at the end of the day, it has to be as much of a guess as the original result. Many of the GPS track websites (mapmyrun.com, etc) use your GPS track and heartbeat data (along with GIS information to accurately determine altitude changes) to provide a better estimate of calorie use, but even they use some sort of model that may not reflect how (in)efficient you really are.
So, at the end of the day, the calories used figure is an informed guess, not scientific fact.
Si
Thanks a lot.
I guess the device is most useful for comparing rides to one another: I worked twice as hard today than yesterday, etc.
Bingo. That’s how I use the “calorie counters” in the stationary bikes at the local gym (I don’t stop until I’ve completed both a minimum time and a minimum “calorie count”).
Wind resistance and rolling friction are both speed-dependent, but they’re speed-dependent in a known way. The device can probably tell how fast you’re going, and estimate the resistances based on that. It’s only an estimate, since it doesn’t know how well-inflated your tires are, or what shape or cross-sectional area you are, or what the wind conditions are, but you’d probably get pretty close by assuming average values for all of those.
Wouldn’t a stationary bike, unlike a meter for an actual bike, be able to measure the actual amount of work done? Of course it depends on how closely the multiplier used reflects your body (as the body is nowhere close to 100% efficient in converting chemical potential energy to mechanical energy, the number of calories you burn is some multiple of the physical work done) - presumably bikes that allow you to enter your age and weight should be at least moderately accurate in predicting calories burned.
In theory, yes, but in my (limited and anecdotal) experience they’re not all that well calibrated in practice.
As an example: my old gym had a row of five recumbant stationary bikes of a particular brand to choose from. Over the years I’d gotten to the point where a good (IMHO) cardio workout for me was to set a bike to a particular resistance level, pump at about 65RPM for 30 minutes and get to the 300 calorie mark. (The target was to get both 30+ minutes and 300+ calories in a session before quitting.) Didn’t matter which bike I chose; they were all about the same.
Then a couple of weeks ago I switched to another gym which had a similar row of recumbant stationary bikes of a different brand. I found that to get the same level of workout I had to get to roughly 350 calories in 30 minutes - targeting 300 calories in 30 minutes was just loafing.
I’m going to hazard a guess that in the exercise machine world there’s pressure to err on the plus side on these things. I mean, if it’s a lot easier to burn 500 “calories” on machine A than it is on machine B, more people are going to purchase machine A. 