Calorie Counting - Apple Watch Vs. Power Meters
 

So even though riding my bike is my favorite thing to do these days, the primary goal of riding is to lose weight (i.e. create calorie deficits). Although the majority of it is done in the kitchen, I'm always curious of my estimated calorie burn during my rides.
My question has to do with electronic calorie counting. Up until now, I kept track of calories burned using my Apple Watch. On my outdoor rides, it uses GPS speed data along with my heart rate to calculate my calorie burn. I didn't have a power meter to compare it against.
Do any of you that have Apple Watches and Power Meters know how well the Apple Watch does compared to a PM and HRM together?
From what I understand, a PM is supposed to be super accurate as measures the energy expenditure thru the work done.
If you have a PM, does the HR even matter?
Just curious.
Thanks!

Power meter will capture actual calories that you put into the road. It does not capture other work that your body needs to do such as keeping the bike upright, etc. Apple Watch measures your move calories via heart rate. If you want an estimate of total active calories you burned, I would use the AW number.

I don't have an Apple watch, but I do have a Fitbit. On rides, the Fitbit tends to underestimate calories compared to the power meter, sometimes by as much as a third. I would assume that it would vary from person to person, though.

Power meter will be within 5% of the total calories used. It doesn't take much energy to keep the bike balanced, drink from your bottle, or look behind you at traffic.

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Your body is only about 20-26% efficient at producing mechanical energy on a bike (reference “Feed Zone Portables, page 6-7). So you need to divide the calories produced at the wheel by 0.20 to 0.26 to get the calories your body burns. Your body burns 4-5 times the energy put into the road.

So a power meter should actually give you a very accurate way of calculating actual calories burned. I haven’t done the comparison to an Apple Watch, however. It probably varies quite a bit from one person to another.

Makes no sense to me to use Calories burned for dietary purposes if you aren't also weighing and tracking every morsel you put into you.

As well, the numbers you come up with for the foods you eat and drink aren't going to be all that accurate. What you see in the Calorie tables are just averages taken from a sample that might have a lot of variance.

So what ever you use, don't get too caught up with how accurate that number is for Calories burned since you can't get any more accuracy with the Calories consumed. It should suffice if your weight is going in a direction you aren't wanting, then you have to change either your consumption or expenditure.

Based on the second sentence of the OP, it sounds to me like he is tracking input. But yes, the proof is on the scale. I found that if I targeted a ~750 calorie daily deficit, I got pretty good results (fast but not too fast). Start with a target daily deficit, see what it gets you, adjust.

Heart rate is not a very accurate indicator of effort, unlike a power meter, which only indicates effort. Heart rate is influenced by everything. It's a variable made out of variables. Power is power.

I use a PM and HRM. The power meter is to monitor the actual work done. The HRM is to make sure I'm not overdoing it in terms of overall load, dehydration, etc, by watching recovery rate (HRR.)

Power meters put it into perspective. People grossly over estimate their calorie burn when using estimates without one and without HR.

HR isn't super bad the flatter the terrain. The hillier, the worse it is. A HRM continues to compute workout calories when coasting downhill and your HR is decreasing and recovering but zero work is going into the pedals. It's this "lag" that makes the computation/correlation of HR to work difficult.

I find when I used to use HR only on my cross bike for gravel rides versus the power meter on the road race bike..........the HR on cross bike grossly overestimated calorie burn and TSS due to this.

I find the opposite on running with HR.

I don't track my calories (i.e. food vs. exercise)... but I do watch the calories between workouts. Like for example if my average ride/workout is more or less 1,000 calories for a given week vs. 1,500 for another week. If I don't change my eating habits between the two weeks, I tend to lose weight. So knowing it does help (as inaccurate as it may be, it just needs to be somewhat consistent)... the question about the Apple Watch vs. PM is just curiosity.
Counting calories reminds me of being in school and calculating the distance it takes to stop an airplane during landing. An old instructor summed up the process as "Measure it with a micrometer, mark it with a pencil, and chop it with an axe."

Never compare if you are going to be bothered about getting different numbers. The power meter will be the most accurate way for us to tell. In a laboratory, they would be measuring the composition of the air going in and the composition of the air going out of you as well as some other things. Most of which we can't do while riding outdoors.

For any one ride, your apple watch might be right or off by a ways. However over the course of many rides, the numbers will give you useful information to assess whether you are burning too much or too little. A PM will give you more reliable info about any one particular ride, but over many rides, will that be any more useful for what ever your purpose is? Again especially since Calorie consumption can't be anything more than an educated guess even with tracking every gram consumed.

It's actually not terribly difficult with modern conveniences like apps and it'll give you a *much* better idea of consumption than just winging it; most people in the US would be in shocked denial if they tracked their consumption for a week. The other nice thing about app-based tracking is that you'll have an idea of your macros, too - probably a good idea if you're looking to maintain muscle mass while cutting body fat. One of my buddies did the typical denial mode diet and he dropped a ton of weight... but now he's skinny fat, if you know what I mean.

When I transitioned into office work, I got a little pudgy. I tried a few different suggestions about how to lose weight and nothing worked. Finally I decided to count calories. This was a while ago, apps weren't readily available, I used a spreadsheet. And an HRM. But it was very effective, the pounds melted away, even after things like "don't eat after 8 pm" had failed me.

It's really easy to forget what you've already eaten, and it's harder to make bad choices when everything is visible. I think that's why it worked in my case, in spite of what must have been bad numbers.

1) PMs will always be a better estimate of calorie burn as they measure actual work put into the machine.
2) I still wear a HRM with my PM mostly out of habit and also to capture HR drift.

I asked a similar question in another sub forum here. I’ve found that AW estimates calories burned at about 60% what Wahoo data with HR and power meter does. Wahoo has posted their algorithm for calorie burn but I can’t find Apple’s algorithm anywhere.

I did a 45 min ride today. My Powertap read 434 calories and my Apple watch said 554 active calories and 655 total calories. I suspect that the Powertap is more accurate.

This is exactly what I do. My HRM vastly overestimates my calorie usage by 30-50%.

On the other hand, the HRM is great at measuring how hard my rides feel. The other major benefit is my max heart rate doesn't change as much as my FTP does.

Combining a PM and HRM has shown me that I can put out far more power inside on a trainer than I can outside for a given heart rate

You're saying the AW underestimates the calorie burn?
Everywhere I read, it's the opposite.

Interesting that this is your experience.
I've heard the opposite from many people that first start on the trainer; in fact, I was listening to the trainer road podcast and they were talking about this.
Their hypothesis was that on the bike, you get a lot more cooling than on the trainer so you could put out more power.

I don't bother with calories for anything. I just weigh myself. Enough navel gazing time for me. I find your last assertion to be true for me. The word is "arousal." That's what you have on the road which gets your HR up and what you don't have indoors. My HR always sucks indoors, which makes the workouts harder, HR being hormone driven. Sometimes I'll yell at myself, make weird aggressive noises, That helps maybe 2 beats worth. I ride indoors in a cool room with a big fan, no shirt. Cooling is fine. A lot of the time I get the perfect temperature indoors: 55°.

Since my HR is higher outdoors, it's easier to put out more power, period. I can do stuff outdoors I just can't do indoors, too much pain. Blood flow = oxygen = aerobic power and lactate removal (or whatever).

I agree about the calorie counting. Kinda doesn't matter how accurate the count is as long as it's consistent. You get a number and respond by budgeting calories in response to that number.

For anyone wanting to know more about sports nutrition, calories, efficiency, etc, I highly recommend Feed Zone Portables, the primary purpose of which is recipes for real food for cyclists.

By the way, compared the number from Strava for my power meter and estimated calorie burn, and it looks like Strava uses a 24% efficiency factor. I'm not sure if that changes with age/weight, but it's in line with what I've read.

PM vs HR calorie estimate aside, Apple Watch is complete garbage at tracking speed/distance/hr. On the same ride/hike we usually have 25% distance difference between 2-3 apple watches. Getting a proper cycling computer (garmin or similar) with right sensors will give you immediate accuracy boost. Also if you use Strava, it does a reasonable job at estimating your Power numbers.

I don't have an outdoor PM, but I just got a Tacx Neo 2T which has a built in PM.
These are my numbers from my 20 mile, 1hr 15min ride yesterday:

AW Total Calories - 873 (workout + calories I would have burned if I didn't workout)
AW Active Calories - 743 (workout only)
PM Calories - 501 (I'm not sure who's algorithm this is... i.e. Zwift, Strava, or the Tacx?).

So yeah... quite a difference.
On the other hand, I've tested the AW HR against my Garmin HRM and they are pretty spot on. I've even tested it against a HRM at a gym and the AW is always the same as others.


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Ignore PM calories (because some do it weird) and look at total kilo Joules from the ride. You can just change the label from kJ to kCal and be within a few percent of god's honest truth.

Last week I had some rides that showed this pretty clearly to me. Two rides at like 87-90 F and two rides at 100+ F. On the cooler rides my speed was about the same for a given heart rate zone as I've seen over the last several months. On both rides over 100 F my speed was about 2mph lower at approximately the same heart rate. I interpret that extra heart rate load as what it took my heart to deal with the heat, and I have a hard time believing that my body was burning the same calories just pumping water to my sweat glands that it would have burned if that same heart rate work product had generated those extra 2 mph of speed on the ride.

To the OP: as long as you recognize that there's a considerable margin of error in all this, the AW data can still be useful. There will be error in your estimation of what you eat, there will be error in the estimation of your basal metabolic rate, there will be error in tracking how many calories your body burned through exercise above your basal metabolic calories burned, etc. All of this means that whatever goal you set for daily caloric deficit will only be a target, and whatever actually happens will not be accurate, but hopefully in the ballpark. Only time and the scale will tell how close it was, and that's all we can do. Unless you spend 24 hours a day rigged to scientific measuring equipment, there's just no better way.