I've read somewhere online that Polar claims their HRMs to be +/-15% accurate on calories burned when compared to controlled laboratory measurement.

I wanted to compare mine with what I get off my Road Machine since the power/speed curve is supposedly pretty well defined and I can back-calculate calories from that.

After riding yesterday, I jumped on the trainer for 40 minutes. The HRM (Polar CS300) says I burned 612 kcal in that time, but the results from converting my average speed (14.4 mph) to calories via the Road Machine power curve (P=5.24482*S+0.01968*S^3, P=134.3 W for 40 min) gives me about 77 kcal*, which doesn't make any sense**. Am I using the wrong conversion from watts to kcal? I'm using 1W-h = 0.86 kcal.

Anyone try anything similar? Is there any way to get a reasonably accurate ballpark of energy expenditure w/out a power meter?

Thanks.



*Assuming about 25% efficiency between body/bike/trainer, that's up to 308 kcal.
**Neither does the 2000 kcal spent over 2 hours and 20 minutes which gives me a sustained power output of 246 watts. That's not right. I'm pretty slow and wimpy.

My brain hurts

Sorry. I get carried away.

Distilled: HRM says 612 kcal for 40 minute effort, trainer says 77 kcal (or 308 kcal if you factor in some magic number). Which one is more accurate... errr... I mean, which one is less inaccurate?

They both sound off. 612 is a lot unless you were working pretty hard, and 77 is ridiculously low. 308 may be closer but sounds a little low. I mean, it's really impossible to say without knowing your weight, fitness, effort, etc. but I would go with the polar number before the trainer number (77).

If you weren't riding too hard, the 308 sounds rightish. I mean, it's all approximation anyway, so if you're close you're close. I know for light effort cycling I burn something like 10 kcal/minute and with moderate effort closer to 12-13 kcal, but I'm a big guy (215 pounds), so your expenditure should be a bit lower. If you were really pushing very hard the whole time, I could see the polar being right, but even at my weight it's hard for me to burn 15 kcal/minute.

To burn 612 calories in 40 minutes, you have to be working pretty hard. That's 936 calories per hour.

For me, I'm working close to a TT level effort to hit 900 calories in an hour, or more like 300 watts.


134 watts for an hour would be more like 300- 400 calories, or 200- 260 for the 40 minutes.

+1.

Those number sound reasonable.

Thanks for the replies. For reference, I'm 6'3", ~195lbs, and in moderate shape. I thought it was kind of suspicious that the HRM said I burned around 1400 kcal during a 1 hr, 40min ride.

Any Polar users here have much luck with the Polar Fitness Test getting you more accurate results?

The fitness test allows it to have some basis for judging your relative fitness, otherwise HR numbers by themselves are meaningless. My LT could be your max HR or vice-versa. IIRC the fitness test on my polar basically established a VO2max estimate as well as an HR range so it had a better idea of where your HR was relative to your limits. If you haven't at least done that I wouldn't even think the calorie numbers would be in the ballpark.

Calculating calories burned based on HR is slightly more accurate than guessing, but not by much.

I agree it's not great but it's certainly better than calories/mile or calories/time. HR is a proxy for effort, so as long as it has a good idea of how hard you are actually working, it will at least give you an idea

Yesterday was the first use. I pulled it out of the package after work, read enough of the instructions to get it running, fed it my age/height/weight, then took off. I'll re-do my ride from yesterday and see what changes after I do the fitness test.

Funny, we had a rainy day yesterday and I did exactly the same thing on a KK trainer. I have a spreadsheet that I use to calculate power and energy used. Your 134W is correct and if you held that for 40Min your energy output would have been 322kJ. A common approximation is to just use the kJ figure as a reasonable estimate of your calories burned. If you assume 25% efficiency you would have burned 308 Cals as you assumed.

I use a Timex HRM and for my trainer ride the Timex estimate was 7% high. It sounds like the Polar is a little more optimistic.

I've tested mine over a dozen times and the HRM estimate ranges from 6-17% higher than a power based estimate. Without going into a lab it's hard to know which estimate is more accurate but for me it's certainly much more accurate than a guess.

I have no idea what that formula says but where does it factor in bodyweight? Clearly:

• a 250 lb person burns more calories per hour than a 150 lb person.
• a 250 lb person w/10% body fat burns burns more calories per hour than a 250 lb person w/20% body fat

I imagine if there was, power meters wouldn't cost do dang much.

I have a Polar 200 cadence heart rate/cyclometer. I'm 37, 5' 7" 138lbs. In the Northern California region there was a challenge put out of how much you could climb in an hour starting from home. In 58 minutes (not including a couple red lights) I got to the top of the steep section of a local 8 mile climb, Page Mill road. The grade is fairly inconsistent as it goes from 7% or 8% to over 15%. When I stopped and checked, the Polar indicated 640 calories. Most of my work day rides are 1.5 to 2 hours, 23-28 miles in length. A light day will be 950-1000 calories and a heavy hill repeat will be 1200-1300. No clue on how accurate this all is but it's a good indicator if I can justify that apple fritter at Starbucks or not.