Calories used according to the gadgets

By Julii Swindells 27 May 2016 | Comment
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Does the Poolmate 2 calculate calorie expenditure correctly?

As soon as you have two plus gadgets you can start testing whether they agree with each other

My Polar (FT4) heart rate monitor reports elapsed time, heart rate, and calories used.
My swimmovate Poolmate 2 reports elapsed time, laps swam... and calories used.

When I wore both the FT4 & the Poolmate for a 51 minute swim session, they came up with identical elapsed times (and credible HR numbers in case of FT4), but very different numbers for calories used (266 for FT4 and 502 for Poolmate2). So which one to trust?

The difference must lie in which formula they use. Swimovate said when asked about what formula they use "We calibrated against lots of swimmers in the pool" which was pretty pants, really. The swimovate gadget (in purple) only asks for the swimmer's weight & the pool length, which is pretty minimal info. Polar doesn't say exactly which formula they use, either, though.

To calculate calories used, Polar FT4 asks for gender, weight and age: all of which are part of the most popular and credible looking formulas online to calculate calories used from heart rate (StackExchange). Note that being young, male & hefty means burning a huge amount more calories than if you're small, female & age 45+. Even if both people perceive that they are working to the same effort level. For example, imagine 3 alternative Me's. Real one (48yrs old, 64 kg). Young Julii and young tall and male Julii. This table shows their calorie expenditure when each version of Julii is working at a constant 83% of maxHR for 50 minutes, with an avg VO2max for age/gender each time. Now you know why teenage boys eat so much and why middle aged spread is real.

Calories used by different versions of Julii
Real Julii 432 kcal
20 yr old Julii 472 kcal
Julii as 180 lb 20 yr old guy 757 kcal

Can the Polar FT4 or its calorie estimates be trusted?


To test whether the FT4 gave credible calorie-burning numbers, I wore it on a run along with a Garmin Forerunner. The Garmin also reported heart rate data, but at a very fine resolution (every second). Then I used the StackExchange calorie burning formula (above link) to try to verify if the FT4 said credible calorie expenditure numbers. The HRs for Polar FT4 & Forerunner matched very well (within 1-2 beats). The Polar said 403 kcal, the Stackexchange formula (relying on Forerunner heart rates and all of my same personal statistics) said 536 calories So kcal about 30% more, and a lousy match. But Stackexchange result wasn't double the FT4 value, which Poolmate2 suggested.

So, in conclusion, don't trust any of them!! :) Not for the bells and whistles rather than core functions, anyway (calorie counting is an add on). Poolmate is fine for counting lengths and Polar fine for HR and Forerunner fine for mapping route, distance and relative effort at specific points in the landscape... but I wouldn't trust any of them for counting calories. Yet, I think they are still useful in a relative sense. If Poolmate2 says 250 kcal after one session & 500 kcal used after a later session, I'd be right to decide "Wow, I feel twice as tired this time!"

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About the author

My name is John Swindells and I'm a keen recreational cyclist with a preference for long one-day rides. I've also previously dabbled in time trialling and cyclo-cross. See more of what I get up to on Strava!

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