New Study finds the Apple Watch more accurate than Fitbit when it comes to measuring aquatic exercise

Apple Watch swimming and water workouts

Many studies have been done over the years to establish the accuracy of smartwatches when it comes to measuring heart rate and step count. Most of these studies have been focused on traditional treadmill or outdoor walking/running type tests.

A new study examined the association and agreement between criterion methods and the Apple Watch Series 4 and Fitbit Charge 3 for recording step count and heart rate when exercising in water on an aquatic treadmill.

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Participants were submerged in an aquatic treadmill pool for the study and completed 3-min exercise bouts at intensities that corresponded to a comfortable walk, brisk walk, jog, and running.

A polar T31 chest strap recorded heart rate, and a high-definition digital camera was utilized for recording step-count.

Significant associations were observed between criterion methods and the Apple Watch. For heart rate measurements, an R-Square of 0.99 was observed, and for Step count, the R-Square observed was 0.87.

The Fitbit charge 3 fared well but not as well as the Apple Watch.

Apple Watch vs Fitbit Swimming accuracy

In the case of the Fitbit, R-Square of 0.72 was observed when recording Heart Rate, and the R-Square was 0.83 for Step count information.

The mean absolute error and relative error (%) for recording step count were 19.8 (7.4%) in the Apple Watch and 21.4 (8.5%) in the Fitbit and 0.90 (0.76%) in the Apple Watch and 4.2 (3.0%) in the Fitbit for recording heart rate. 

Both devices displayed a reasonable level of agreement for recording step count and heart rate when exercising in water. 

Apple Watch performs well in its attempt to quantify the complex motions of watery workouts. swim stats on Apple Watch and iPhone

The watch uses the gyroscope and accelerometer to track the motion of your strokes, but in open water, it can leverage another sensor: the GPS chip. If you’re swimming in the ocean or a lake, your watch uses that to determine how fast and how far you go.

The accelerometer measures motion, and the gyroscope determines how many degrees the watch is rotating per second. Together, those sensors help Apple figure out the stroke type.

Beyond the swim tracking mechanics, the Apple Watch does a beautiful job reading heart rate information even when you are in the pool or open waters.

Linear regression analysis demonstrates the association between each wearable device and the Apple Watch and Fit Bit Charge for recording step count and heart rate.

Results of the study were first published this week in International Federation for Medical and Biological Engineering.

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Sudz Niel Kar
I am a technologist with years of experience with Apple and wearOS products, have a BS in Computer Science and an MBA specializing in emerging tech, and owned the popular site AppleToolBox. In my day job, I advise Fortune 500 companies with their digital transformation strategies and also consult with numerous digital health startups in an advisory capacity. I'm VERY interested in exploring the digital health and fitness-tech evolution and keeping a close eye on patents, FDA approvals, strategic partnerships, and developments happening in the wearables and digital health sector. When I'm not writing or presenting, I run with my Apple Watch Ultra or Samsung Galaxy Watch and closely monitor my HRV and other recovery metrics.


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