Can anyone explain Strava cals vs Garmin cals?

They likely use different methods to estimate calories burned. Both are just that, estimates.

Which Garmin do you use? Some base the calorie estimates on heart rate, others simply use a formula even if it has a heart rate monitor (I believe all the newer models base on HR if available)

One or two people have noticed that the Calorie consumption figures for a given bike ride may well be different to the figures that are calculated using your Garmin or other Logging devices... I noticed this a while ago, and being the kind of numerically obsessed geek that I am, I decided to look into it further.

It is my contention that rather than simply taking the calorie calculation that your HRM/GPS logging device provides, Strava has taken upon itself to try and give what could potentially be a more accurate figure - certainly if the data logging device is a non heart-rate-monitor linked smartphone...

As I'm sure you have all noticed, Strava works out a "Virtual Power" figure for your ride, which is based on the speed you're riding at and the amount of up/down/steepness of gradients you have encountered. So, for the entire ride, Strava has a figure for the number of Watts of power you are "laying down". Therefore, it can also add this up and arrive at the number of total watts expended, which, divided by the number of seconds ridden, gives a total amount of energy expended in driving the bike and you along. (Basic Physics - 1 Joule = 1 watt per second).

So - to take a worked example - my Sunday ride from the 13th October 2013 - http://app.strava.com/activities/88753940 - we have a "Total Work" figure off 1,555 kJoules of energy needed to drive me and the bike around that particular route. Okay, so how do we get from that to Calories. Simples - google tells us that 1 joule = 0.239005736 calories. For the purposes of this calculation, lets simplify a little and just work to 4 decimal places shall we...

Therefore, the actual energy used to move me and the bike around was 1,555 *0.2390 = 371.645 kCalories.

Wait a Minute Mark, I hear you say... Strava reckons that you burned 1656 kCalories on that ride, not 372! ... What's going on ??

Again, it's a fairly simple thing. While the Pedal Cycle is a wonderfully efficient machine, the Human body is a lousy inefficient engine (some more than others - but I'll get to that later!) So, while I pushed out 372kCal of energy riding, I also wasted energy in sweating, in digesting my breakfast, in thinking about what I was going to have for dinner, worrying about if I'd get home in time to hand over a Turbo Trainer to Fran, producing Snot, My heart beating approximately 20,000 times, Talking to fellow cyclists, filling (and subsequently emptying) my bladder and 1001 other little things that the human body does while riding a bike that AREN'T directly related to shoving the pedals round. This is where things go from the directly measurable (because, in fairness, MY power figures WERE from a real Power Meter, rather than Strava's Guesstimates) to the "taking an average figure and hoping it's right"

So, let's call the difference between what you shove, and what you burn to shove it the "Metabolic Efficiency Factor" (or MEF for short, because I can't be doing with re-typing that again and again. Again, a quick Google seems to show that studies tend to put this MEF at somewhere between 20 and 25%... So - I did a little digging...

All this data is based on MY ride information over the last week or so...



I've basically used the Calories and Work figure to come out with the MEF that Strava is using... Allowing for Strava only displaying whole digit numbers, it would appear that the MEF for my rides seems to be coming out pretty consistently at 21.43%. Interestingly, the data also shows that its using the same figure for rides on my Roadbike (which has a power meter on), on the Indoor Trainer (which was taking power figures from the Indoor trainer's calculations of resistance) and from the MTB, where the power figures were wholely down to Strava's own power guessing routine...

Just for the sake of thoroughness, I also decided to have a look at a cross section of the members of the GS MyFitnessPal "club" and see if if this MEF changed (say, depending on Age, Gender, Height, Weight, Calculated Body Mass or any other wierd and wonderful way.)



NOPE. Same figure more or less... and that list covers a fair cross-section of ages, genders and body shapes I reckon... It appears that they've decided on a "one size fits all" approach...

Just out of interest, here's a good bit of data...

Calories 6621, Work 5938, 21.43%... want to know who that was... Laurens ten Dam - the Pro from the Belkin Team..

( http://app.strava.com/activities/86637795 for confirmation )

Now, call me a defeatist, but I seriously doubt that MY metabolic efficiency is likely to be exactly on a par with one of the better Pro's :laugh:



But, you may also have noticed another column in the first little chart I posted... The "Garmin Calories"...

Well, it's self explanatory I suppose - thats the calories that My Garmin read at the end of the ride. One thing to notice is - they're all over the shop... you'd expect them to be all either higher or all lower, or all somewhere near, wouldn't you...

So did I at first. Then I thought about it a little more. You see, a few months ago, I went and submitted myself to one of those "NewLeaf" Metabolic Profiling tests. There's a really good writeup on the DC Rainmaker blog if you want to know more about the test ( http://www.dcrainmaker.com/2012/01/look-at-testing-with-new-leaf-fitness.html ) - mine was much the same, but on a Wattbike rather than a treadmill thankfully.

The end result is a sort of "profile" of my metabolic efficiency factor, broken down into 10bpm "bands" so the Garmin knows that if my HR is showing between 80-89 bpm, i'm burning 6.76561kCals /minute of fat and a total of 9.02081 kCals, whereas between 140 and 149 i'm burning through 0.76040 kCals of fat and 13.80201 kcals in total. It has data for basically all points between 30bpm and something like 240bpm, though if I hit either of those figures, i'm sure my calorie expenditure would be the last thing on my mind.

I guess that all this waffle show is that estimating Calories from exercise is a) difficult b) an inexact science and c) if you give 5 people a chance to do the calculations, you'll get 5 different ways of performing the calculations and AT LEAST 5 different answers.

For me, I Have a sort of heirachy of how much I believe the data... From least belief to most it runs...

MFP's Guesstimates based on time of a "exercise"

Endomondo based on smartphone tracking but no HR data

GPS and HR tracked data

GPS / HR and POWER tracked data on a "Vanilla" Garmin Device

GPS / HR and POWER tracked data on a "NewLeaf Calibrated" Garmin Device

...



But there's a REALLY good rule of thumb to work by... if you've got 2 or more different figures - LOG THE SMALLEST and err on the side of caution.

I should say, it's something I posted in the Strava Cycling Group, so it's sort of cycle-performance oriented... but the technical stuff still should apply...

Oh my god Mark, what a response,,, Let me take a min to read.

I use a Garmin forerunner 110, with a HRM, only entry level model, but had it nearly 2 years and suits me

EDITED to say, read the reply, thank you, interesting figures

The 110 does use HR based computation for calorie estimates.

As in, it does take my HR into account in calculating calories burnt? Sorry if I'm being dumb.

Yes. (not being dumb).