So ive noticed since starting TR that the KJ’s burned seems rather low for my perceived effort BUT my only marker before was what Strava told me. Do you guys think Strava massively overestimates, TR underestimates or somewhere in the middle?
If you are using a power meter, then you should just use the kJ that it reports on TR.
If you are using virtual power, it will vary, but be pretty close.
Never trust Strava’s data on pretty much anything (except elapsed time and even that can be wrong with GPS drift). It’s wrong in so many different directions simultaneously.
According to my power meter Strava overstates calories by about 10%
When syncing a power meter ride from Garmin Connect to Strava, Calories shown are identical for me (or off by less than 1%, usually because of the way strava uses moving time instead of total.)
Comparing this with non-PM rides from GC, the amount of Calories in Strava seems to be overestimated by 10-20%, especially for low intensity rides. So it’s still a reasonable number if you are looking for a ballpark figure. If you’re doing calorie counting/MFP on the other hand, it could be enough to kill your deficit for the day
Using kj underestimates calories by a fair amount. Nate posted an article with froome’s lab results and he has 23% pedaling efficiency. They explained that his efficiency was highest than most, so most if us average joes are closer to 20% than 25%. Stravas calculation works out to a 22 or 23% pedaling efficiency which is more accurate than tr’s 25%.
I did a 52minute outdoor ride the other day. Strava estimated 830 calories, TR estimated 495!
Edit: with PM & HR data
I definitely take Strava’s calories with a pinch of salt. Especially if it has no other metrics to use like power or HR. With neither of those I’ve seen it overestimate by 100%! With HR alone its pretty high. Based on the TR rides with power its much more what I would expect.
What was your average power on that ride, unless you’re a real beast, burning 830 calories in 52 minutes would be a pretty impressive feat. That’s like 1000 calories an hour almost - depending on your efficiency you’d have to average (not NP) 240-260W.
NP according to TR was just 207.
For me strava consistently overestimates calories.
When you say a “fair amount”, you’re just talking about that +/- 10% difference between 20% efficiency (vs ~22.5% Strava uses) or a 25% efficiency (again vs 22.5%), right? Or are you thinking there’s more than that?
Yes, it is just the difference in the 25% efficiency used to say calories = kj when we are somewhere in the lower 20% range. While it is a few percentage points in efficiency, since you are dividing by the efficiency to determine total calories burned, it makes a pretty big difference in total calories.
And it looks like since there are actually 4.18 j/ calories that their efficiency works out closer to 24% So if you calculate calories burned using 22% efficiency, you can get a large difference.
And looking at my most recent strava indoor file, it is not entirely based on the power, so I am not sure how they are calculating calories when it uses both HR and power.
I just did a 780 kj ride, and strava is saying I burned 766 calories. If you calculate based on a 22% pedaling efficiency, then my calorie burn jumps up to 850. a 2 % difference in efficiency ~ 9% difference in calories. I still think I’m even overestimating my pedaling efficiency since as I was saying before, if Froome’s excellent 23% is a big advantage, I’m guessing most other grand tour riders are around 22%, putting most of us in the 20-22% range.
For most of us though underestimating calories burned is not a bad thing, so it is much easier to just say kj = calories burned but the right answer is far more involved.
For most people, they need an underestimation more than an overestimation
I’m comfortable recommended kJ as the best representation of caloric expenditure given that all of the other options are bad.
Side note: Slightly off-topic from the OP, but for outdoors, keep in mind full body calorie burn, not just kj from pedal-only power… especially for MTB or cross where you’re putting in a ton of upper body and core work in as well. This is obviously not even remotely accounted for with a power meter.
I’m doing a medical check once a year, it’s a ramp test with VO2 and VCO2 measurements. That gives me the data on both power and respiratory gas volumes, I think I could nerd out a little and do some analysis - it should be possible to derive a pretty accurate calculation of calories burned and compare it to the power meter data. It’s going to be a n=1 case study, but it could help to answer some of the above questions.
Just like calorie burn you should take Strava’s power estimates with pinch (or ten) of salt. Especially for group rides.
I’ve noticed massive variance for low intensity but not so much for moderate intensity. For comparison:
(My PM tracks within 1% of my KickR and was serviced in June)
A flat ‘recovery/woosah’ ride outdoors.
PM = 960kj~ / Strava = 800 calories
Strava showing 20% less
This afternoon’s ‘Tempo’
PM = 2,254kj / Strava 2,250 calories
Less than 0.002% variance
I eat to Kj and maintain, mostly…
I use kj indoors, but Garmin/ Wahoo data based on HR and Power (well when I have power) outside. Wind, rolling resistance, weight of extra stuff (saddle bag/ pack back/ food/ phone/ keys etc) must come into play? Well that’s my logic anyway, on top of pedaling efficiency.
But I still find strava overestimates compared to Garmin or the Bolt.
TR calculates kJ from the power meter if available, so it should be reasonably accurate. I’m not sure if Strava calculates calories from kJ or from HR. I use the TR number as my baseline. A man with two watches never knows what time it is, so pick one.
I think the kJ -> kCal rule of thumb is kCal ~= 1.08 * kJ, based on “average” efficiency. Average is the thing everyone compares to but no one is or wants to be.
Calories in food often vary 10% or more from the label, your efficiency can vary day to day, your power meter varies, your body’s water content varies, probably seventy other things vary day to day or week to week. So, as @stevemz said, it’s probably best to use kJ and underestimate and watch the scales over time to move (slowly) toward or stay in a target range.
If/when you have power outside why would you omit its absolute measurement of work done? Unless I’ve misunderstood?
Quite rightly, all those variables will effect your output but 200w on the trainer vs 200w outside in to a headwind with the kitchen sink on your back isn’t going to burn any more energy and might feel harder but isn’t.
But those would effect efficiency, wouldn’t they? I don’t know, maybe my logic is wrong, but using that and calorie counting, I’m able to maintain weight.
I’ve taken the results of 2 ramp tests with gas exchange measurements and compared it with the data from the power meter (just to keep the consistency I used the data I received from my doctor rather than my own PM - both tests were done on Cyclus II ergometer). I’ve changed my training quite significantly between 2017 and 2018, I think I may post the full analysis in a separate topic. But for the purpose of comparison, here’s how kilojules calculated from power output track to actual energy expenditure as measured from VO2 and VCO2 volume:
horizontal axis is Watts, vertical axis is kcal/kj per hour
My FTP was approx 300W in both cases and it’s clear that for the usable power ratio, between 50% and 100% FTP, all 3 lines track pretty close. As expected, the moment we go above FTP and anaerobic contribution is higher, the oxygen uptake goes through the roof.
This was an indoor test, I think that an outdoor energy expenditure with better cooling would potentially be lower, on the other hand riding in cold conditions could require more energy. In any case, that chart proves to me only that for nearly all intents and purposes using kJ to match the kcal is pretty much good enough.