Apple Watch Calories vs. PM5 Calories

[thelastr12t]

Hi guys, new to C2. Use a Model D with PM5. One thing I notice is that the calories counted by Apple Watch (Rowing Workout) are quire a bit lower than the calories shown on PM5. Any ideas where the difference comes from? (With other Fitness equipment like treadmill or elliptical, the differences are quire smaller...).
Thanks. Oliver

[Citroen]

Both numbers are mostly meaningless.

C2 calories work if you burn 300 doing nothing, weigh 80Kg and are 25% efficient turning calories into work.

Most folk won't match that model.

[Ombrax]

Estimating calories used is at best an imperfect science.

Given the choice between an Apple Watch (which I assume is basing it's number on HR) and a C2 PM, I would guess that the PM5 would have a better estimate of the actual work that was done in a rowing session. Whether or not the PM's estimate of the calories used is better is tough to say.

IMO it's best to use calorie guesstimates as a relative comparisons of workouts (e.g. shorter hard workout vs medium length medium effort vs long workout at lower effort), not as an absolute gauge of how many of the three donuts x 226 calories you ate at work you've managed to work off.

[Ollie Russell]

Ive never used an apple watch, i use a fitbit2. If its anything like the apple watch then i would follow the apple watch out of the two. Im assuming apple watch is the same with regards to information you input; age, weight, height, gender and bodyfat. Again im assuming, but these must be taken into account when the apple watch/fitbit give calorie output readings...surely? So the watch must be more accurate as the pm5 requires no information to give a calorie output, although the pm5 is task specific. I dont see the harm in working out the average between the pm5 and apple watch...perhaps that would be closer to what you have actually burnt.

I have never relied on cardio equipment for calories burnt, but if you are using the same gym equipment then it can be a guide for how hard you have worked. Logging your diet and gym sessions and tracking weight would give you all the information you need.

[jamesg]

The C2 estimate is as good as any, since it does actually make a real measurement of the work we do on the handle.

I usually lose about ½ kg in a session of 20 minutes, so produce around 250 kCal heat. Which is about the heat content of 1 ounce of butter or oil, and tallies with the C2 readings.

[Nomath]

My scale has a resolution of 0.1 kg. On most days I loose 0.3 kg in a 5K row of 20-23 minutes and 0.6 kg in a 10K row of 42-45 mins. The mechanical work amounts to about 210 kJ for 5K and 405 kJ for 10K. The known efficiency of the human body in converting its fuel stock into mechanical work is between 20-25%. So the burned fuel stock should produce a factor of 4-5 more than the mechanical work, roughly 950 kJ for 5K and 1800 kJ for 10K . Carbohydrates have an energy density of 17 kJ/g and body fat of 39 kJ/g. Because these rows went at a fairly high intensity, an average of 20 kJ/g is plausible, resulting in an estimated 50 grams of fuel burned for 5K and 100 grams for 10K.

The conclusion is that more than 80% of the observed weight loss is due to water leaving the body either by breathing or sweating. It will be quickly replenished from the tap or by cold drinks from the fridge.

It may well be that in summer or in meditteranean countries the weight loss is significantly higher.

[Allan Olesen]

The conclusion is that more than 80% of the observed weight loss is due to water leaving the body either by breathing or sweating.

As far as I can see, that was actually the premise, which jamesg used in his calculation.

If the body's efficiency is 20-25%, this means that 75-80% of the combusted energy is not going into work, but will eventually end up as heat. The body will need to dissipate this heat. If we assume that most of this heat dissipation happens through sweating, and we know how much water was evaporated, we can use the evaporation heat of water (approx. 2400 kJ/kg) to estimate the dissipated heat.

There are a lot of uncertainties and assumptions in this estimate, but the line of thought is quite clever. I have never seen this method used before.

[jamesg]

C2 considers us as fuel cells, efficiency 25%, adding 300 kCal/h to allow for work done not on the handle.

I assumed all weight loss is steam (so 500 kCal/kg) and fuel weight negligible, which assumptions tend to cancel.

[Allan Olesen]

300 kCal/h to allow for work done not on the handle.

...and to allow for being alive...

This has come as a surprise to me, but apparently it is kind of a convention among exercise trackers to include our basal metabolic rate (BMR) in the calorie estimate of an activity. If I start an exercise activity on my Garmin watch and then go sit on the couch for an hour, that activity will have counted around 88 exercise kcal per hour - the number of kcal I would have combusted anyway.

This makes absolutely no sense to me. I want to track the number of kcal I spend on top of my BMR, but apparently that is not how it is done.

(Luckily, my watch also has a tracker for actual active calories, which does not include BMR. But that number cannot be seen anywhere in a logged activity, so I have to mentally subtract my BMR.)

[/rant]

[Nomath]

The armchair metabolism of 88 kcal per hour is remarkably close to 100W (88*4.2*1000/3600 = 103 J/sec), often taken as the default base metabolic rate.

The base metabolic rate is hardly affecting my rough calculation based on weight loss after a 5K or 10K row. The weight loss during a night sleep (6 hrs) before taking a pee is about 0.2 kg. If 100W is plausible, most of this nightly weight loss is due to moisturing the air while breathing.

[Allan Olesen]

The armchair metabolism of 88 kcal per hour is remarkably close to 100W (88*4.2*1000/3600 = 103 J/sec), often taken as the default base metabolic rate.

Pure coincidence. It is a function of the body weight I have entered in Garmin Connect.

You can call it insignificant, but it still adds up to around 30% of those 300 kcal/h, the PM5 puts on top of the kcal consumption directly calculated from the measured work.

[Nomath]

I didn't call the basal metabolic rate insignificant, but the weight loss due to the BMR (~0.2 kg/6 hrs = 0.025 kg/45 min) compared to the observed weight loss in a 10K session (0.6 kg/45 min).

For the 5K and 10K sessions that I analysed, roughly at 145W, the PM3 displays 796 Cal/hour. The Calorie Calculator corrects this to 783 True Calories/hour burned for a body weight of 76.0 kg. A BMR of 88 Cal/hour contributes much less than 30%. I think you forgot to take the muscle efficiency into account.