Calorie Burn Accuracy

First time using the F7XSS paired to the rowing machine on Wednesday.  Obeservation:

Garmin calculated 943 calories burned, compared to Concept2 PM5 calculated 1807.  Note that I was wearing my Garmin HR strap (which I do for all rowing sessions, weight training, and HIIT workouts) and was paired to both the FENIX7 and the Concept2 ERG.  Average heart rate was within 3bpm between Garmin and Concept2 for the duration of the row, yet Garmin FENIX 7 estimates half the calories burned.

All other metrics collected by the FENIX7 were accurate, to include stroke rate, distance covered, 500m split pace, and power.  Attached image is the same rowing session from each data source (Garmin FENIX7 on left | Concept2 PM5 on right).

Anyone else noticing this with fitness equipment paired directly to the watch?

  • 2 weeks ago my Fenix 7 stopped recording calories burnt correctly when doing spin. It’s gone from 500 to 90 calories burnt, all other stats are correct. Am sure it’s a setting that’s changed when I hit start indoor bike last time but not sure what it was. I’m not using my HR strap btw 

  • Based on the power data, which may or may not be accurate as it is a calculated value in Concept 2, with gross metabolic efficiency rating of 20% you'd get a calorie burn of around 1450. Normally gives me about 550 calories per hour Kayaking at just under 3min/500m and 600 calories per hour cycling with a power meter at Zone 2. 

  • Except that it is extremely unlikely you could get to your lactate threshold on a bike at 152w...unless you are a lounge lizard :-) 

  • It's interesting. I just put in heart rate data and workout duration into a calorie calculator and for someone who is 85kg and Vo2 of 42, calories burned @160bpm would yield a burn of around 1800 cals. Bpm of 120 = burn of 1100 calories. So, based on heart rate alone you would expect more than 900 cals in 2 hrs at 120bpm.

    But using formula: Calories (joules) = Power x Time (hrs) x 3.6, you would have burned 1080 Calories.

    150 x 2 x 3.6 = 1080.

    So, whether using heart rate or bpm, you would have burned around the same.

    That is comparable to a recovery ride on my bike in power zone 2. I have power meters attached to each pedal - at average heart rate of 125bpm and power of 130 watts I burn 560 calories in an hour 1120 cals in 2 hours. Unless you are super fit, it doesn't sound feasible to have a heart rate of 120 bpm whilst sustaining 150 watts for 2 hours on a rowing machine. I say this because you are using your arms/upper body primarily which would require a lot more work from your cardiovascular system compared to legs which are much stronger and larger muscles. That means a comparably higher heart rate. Maybe you are super fit! If so, good for you Blush.  I think the feedback that the power meters on a rowing machine are inaccurate is probably correct - unlike on a bike, where the measurement is highly accurate. It's the power measurement, not the heart rate data which is the problem. 

  • There's no such thing as accurate calorie burn data from either an exercise machine or a fitness tracker. There's a reason none of these devices are considered medical devices, and that's because they would have to be within 20% of actual values to be considered a medical device.

    Direct calorimetry is obviously impractical, but there have been studies done where direct calorimetry has been compared to fitness tracker estimates and the estimates range from 50% - 200% inaccurate, generally (but not always) being too high. 

    If you really want a decent estimate of activity calorie burn the best you can do is to try and calculate it manually after rigorously tracking weight and caloric intake over a longer period of time, as well as making sure you have a good idea of RMR, preferably through indirect calorimetry 

  • Based on your excercise I would say the value from FENIX7 is way too low, definitely wrong. The other one seems more correct/realistic. (I did a PhD in exercise physiology, and am a cyclist.)
    I also have a FENIX7 (for 4 months now), and I am also shocked how the "calorie burn" estimated by this watch is wrong (low by about 2-fold, I would say).
    Before this watch I had Polar watches (high-end models) since 1987, and they were giving me pretty accurate values for energy expenditure - about 2x what I now get with my Garmin FENIX7.