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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Love working out in the water? A new study says to use an Apple Watch for best results
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.
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. 
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.
