A quick running puzzler: Calories burned

Posted by Dave on August 11, 2011 | 10 Comments

I’m still on vacation, but I thought I’d throw out a quick puzzler for you. For the past two days on my run I’ve completed one of the most grueling miles I’ve ever faced; it includes a 25 percent grade for over 2/3 of a mile, for an 870-foot elevation gain. Here’s the elevation profile of the run:

As you can see, it’s quite dramatic. Yet if you look at the record my GPS trainer (a Garmin Forerunner 305) made of the run and saved on Garmin’s website, it actually indicates I burned fewer calories than I did on any other mile:

On the downhill and flat Miles 1 and 2 it says I burned 152 and 171 calories respectively, but on steep uphill Mile 3, it says I only burned 131. Why? I’ll leave this one for you to puzzle out in the comments. One bit of information that might help: I don’t use a heart rate monitor.

Comments

10 Responses to “A quick running puzzler: Calories burned”

  1. Toni Marano
    August 11th, 2011 @ 7:17 pm

    From my understanding calories burned are directly related to the intensity level or HR level. How would you really know if you don’t wear a HR monitor?

  2. Toni Marano
    August 11th, 2011 @ 7:18 pm

    btw 25% grade is gnarly!!

  3. Chas
    August 11th, 2011 @ 8:27 pm

    Dave,
    The 305 (or mine, at least) probably calculates caloric output based on your bodyweight (if its programed in your training menu) and your average pace for a given lap, provided you don’t use the HR monitor. So, since Garmin doesn’t know exactly hard hard your heart was beating during that 3rd lap, it just sees a slower pace and assumes you are easing up your intensity. I guess it would be a lot to ask for the Forerunner to cross-reference that with the elecation profile and give you the credit (calorie-wise) for your hard climb. This is all speculation on my part. Anyway, I’m sure the caloric output would show very differently in the splits if you wore the HR monitor like Toni mentions.
    Personally, I don’t like the feeling of the thing on my skin. :)

  4. Adrian
    August 11th, 2011 @ 11:23 pm

    Has anyone found that the calories burned really does vary with heart rate or elevation? I haven’t done a detailed test but a quick check of my slow, flat road runs show that the calories burned is roughly the same as grueling trail runs of the same distance even though my subjective exertion is a lot different.

  5. Roberto
    August 12th, 2011 @ 12:35 am

    Of course calories burned varies with effort (i.e. heart rate). Those calorie counter functions are merely an indicator, and are in no way scientific.

    The principle is easier to understand in the context of swimming, in which efficiency is everything. Some years ago, after injuring myself running, I took up swimming regularly. I swam every day for an hour, doing intervals, and got to the point that I was a pretty decent swimmer. Rather, I was a pretty fast swimming, for a guy who hadn’t swum competitively. I did that mostly on aerobic fitness – I was able to drag myself through the water tirelessly, and that’s one way to do it. The other, better way, is to be an efficient swimmer (or runner). I am 6’0″ and weigh around 165 lbs and I guarantee I burn fewer calories running than you do, if you measure up roughly the same.

    Further note on swimming: after I started that swimming program, a guy who worked for me asked if he could join me during my lunchtime sessions. I set him up as a member of my athletic club (which rented the pool lanes) and he started swimming. He was an absolutely awful swimmer – looked like one of those old-style hand-cranked food mixers when he swam – but he thrashed his way up and down for the same hour I was doing my intervals, and after a few weeks, he was absolutely ripped. He had been getting a FAR more strenuous workout trying to stay alive in the water than I and my training partners had doing our intervals at three times the pace. Interesting.

  6. Chris
    August 12th, 2011 @ 1:30 am

    There is only one way to accurately measure calories burned. A probe….

    Since that may not be the way most want to estimate their burn, estimate algorithms are used. Garmin 305s are very low powered devices. Thus they try to perform very basic calculations.

    Garmin 305 has two basic calorie burn calculations:
    1)Heart Rate Calculation:

    This one is essentially the integration of your heart rate augmented by weight and time. Your level of effort is your calories burned. This metric was designed and validated by FirstBeat. The knock on this was that it has only been validated against elite athletes. however it is a simple algorithm with a 5-10% error (Not Bad!!)

    2)Speed/Distance Algorithm: In the absence of a heart rate monitor the 305 defaults to a Speed/Distance algorithm. It weighs distance much stronger than speed (i.e. total time is more important that how fast you run). The algorithm is augmented by weight and age. A very simple algorithm that has a typical error of 35-40%. This is essentially the same algorithm a treadmill would use.

    Interestingly, neither take elevation into account. This is primarily due to the fact that the Garmin uses a baro alt report. baro is only good to about +or- 50 feet (Not good!)

  7. Christiane
    August 12th, 2011 @ 6:29 am

    With my GPS device (Garmin FR405), calories burnt seem to vary with speed – the slower I go, the less I burn. For hilly courses or even mountain trails, you either keep that in mind or use a hrm once to get a feeling of just how much hard running increases your calorie count.
    What ‘m still puzzled about is how much calories burnt goes down if you practically do a run/walk (which I had to recently, following injury). Might the formula weigh performance too much and base rate too little?

  8. nwaber
    August 13th, 2011 @ 7:50 pm

    I’m willing to bet that it just uses speed/distance, based solely on horizontal movement (no calculation for slope distance or difficulty) unless there is HR monitor input. All it sees is that your pace was something like 36% slower over the distance, and it figured that you were therefore running with something like a third less intensity (since, of course, there was no HR monitor to tell it otherwise).

    While a GPS-based slope difficulty algorithm could be nifty, I think that it would be of limited use. One would have to be able to program in everything from ground condition (ie. running up a sand dune vs. running up a paved hill) to how much you have trained for hills. Though a physiologist may shoot me down and it may turn out that a native Netherlander may burn just as many calories running up a hill as your average sherpa.

  9. mtnbkr1
    August 13th, 2011 @ 11:35 pm

    Chris is pretty much spot on.

    The Garmin 305 even when equipped with a HR monitor uses a different algorithm to calculate calories. In most cases it gets you close, but as you’ve discovered it can be wildly inaccurate in certain circumstances. The 305 uses your profile (age/weight), speed and distance to determine caloric expenditure. Even if you have the HR monitor, at no time does the 305 use that data to calculate caloric expenditure. (For the record, the reason changes in elevation are not used in the calculation is because until your upload your data to a site like garmin connect, your elevation data can be off quite a bit too. This is less of a concern in the Edge series of devices that has a barometer to measure elevation. The Forerunner series doesn’t have a barometer.)

    The newer Garmin Forerunner devices (405 310Xt and the 610) will use your activity HR (using the algorithm from Firstbeat) and will provide more accurate results than the 305.

    What we are really talking about here is that these devices work OK in some circumstances, better in others, but are by no means entirely accurate. Considering that the devices are mass produced and the calculations presented have to work for everybody from the 250lb person running their first mile, to the 135+ professional runner, it’s easy to see that there are going to be wild fluctuations in the calculations. On **average**, they work ok.

    Calculating calories burned is an activity that isn’t for the faint of heart (pun intended!) and there are many more factors that play into it than people care to know, let alone measure. For more info on the scientific side, see this whitepaper – http://www.firstbeat.net/files/white_paper_energy_expenditure_estimation.pdf

  10. David
    August 14th, 2011 @ 2:30 am

    Two problems at least with the Garmin. First, one could calculate work to lift your mass that height against gravity and those calories are above just moving. Second, it must report “marginal calories” or those above just sitting. Otherwise, i am certain that a 15 minute mile burns more calories than a 5 minute mile (but the 5 min mile plus 10minutes waiting for you will likely burn more.)