Accuracy test: Calories from high end HRM on a bike (HIIT and steady state)

NorthCascades

I wanted to see how accurate my Fenix 3 HR is at estimating calories from HR, so I tested it against a power meter. I started off fresh, did a "steady" warm up, a high intensity hill repeat workout, and then a cool down while I was fatigued from the HIIT. I used the lap button on both devices to split the ride up, so we can compare each portion.

I used a chest strap (Garmin HRM-Run) to measure my heart rate. The Fenix 3 knew a great deal about me: my height, weight, sex, age, bike-specific VO2max, FTP, LTHR, resting and max HR, 20 minute power record, and workout history for the past year, plus sleep, steps, and other things. It ran a GPS and a barometric altimeter during the ride so it knew distance, speed, and elevation during the ride too.

Warm up: 11:53, 125 bpm, 120 w, 85 kilo-Joules, 115 calories estimated from HR (35 % over)
HIIT: 10:48, ~160 bpm, ~450 w, 118 kilo-Joules, 145 calories estimated from HR (23 % over)
Cool down: 9:37, 129 bpm, 100 w, 57 kilo-Joules, 77 calories estimated from HR (35 % over)

The HIIT part is actually split into 11 laps so I've eyeballed the averages here.

Comments:

HRMs really aren't reliable for estimating calories. Bikes are supposed to be a best-case scenario for HRMs as calorie guessers.

It was closer to the truth for HIIT than it was for "steady" state.

Fresh vs somewhat fatigued didn't make a difference. My average HR was up slightly to produce 17 % less power, and the HRM wasn't fooled. Both of these sections were overestimated by the same amount.

I'll do this kind of test with different types of rides until I sell the Fenix 3.

sijomial

But, but, but - everybody knows HRMs are all-seeing magical energy counting devices! (Yeah, no.....)

I managed to get my Polar FT60 HRM calibrated well enough to be almost identical to a power meter equipped indoor bike (WattBike Pro) - but only under perfect conditions, warmed up, steady state etc etc...

Here's some conditions where it goes adrift:
1/
200 watts steady state for half and hour = almost perfect agreement.
220/180watts five minute intervals for 200 watt average over same half hour and HRM now estimating 100 cals higher (confused by my recovery from the higher intensities)

2/
Lost fitness following chest and sinus infections - power (and therefore energy expended) at identical heartrate now fell by 10%. I would have to re-calibrate the HRM again with VO2 max tests etc... Guess not many people do that regularly.

3/
Here's an example of cardiac drift - steady state 180 watts for an hour. Heartrate slowly climbing the whole way despite power being pretty constant. 145bpm I regard as my HR to produce about 180 watts but by the end of the session I hit 160bpm, mostly due to over-heating.

kcjchang

You'll be surprised the number of folks using power meter still holds HR as gospel let alone the general population, it's not limited to calorie counting. I started training with HRM in 1988 when they became cheap enough while racing in college (~$400 strap and watch monitor w/o data collection just summary). Took a long break between 1992 to 2014. It's amazing the same twisted interpretations are still being pushed after all this time. Just glad power meters finally made it out of the lab at a reasonable cost.

DannyYMi54321

I haven't ever looked at data from my watch in that kind of detail, but I always thought the calories burnt numbers were crap. Back when I was closely tracking food and exercise for a period of 6-8 months about 4 years ago, the watch #s should have resulted in about 1/3 more weight loss than I had in real life.

dewd2

For running, I have my Forerunner 630 dialed in pretty close to reality. It took a couple weeks of trial and error, but I eventually got the settings correct. These days I use the calorie count to ensure I'm eating enough. It is tough for me to maintain my weight when I am training for a marathon (even when I eat every damned thing in sight ).

Having said that, I would not rely only on what my gadgets say. My watch is just one of many tools I use and it gets used mostly as a training aid. Folks need to understand that heart rate has little to do with calories and most of a fitness watch's 'features' are just made up nonsense by the marketing department.