Agreement of the Apple Watch® and Fitbit Charge® for recording step count and heart rate when exercising in water

This study examined the association and level of agreement between criterion methods and the Apple Watch 4® and Fitbit Charge 3® for recording step count and heart rate when exercising in water on an aquatic treadmill (ATM). Sixteen healthy participants (13 females and 3 males) volunteered to take part in this study. Participants were submerged in an ATM pool to the level of their xiphoid process 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 (HR) and a high-definition digital camera was utilized for recording step count (SC). Significant associations (p < 0.001) were observed between criterion methods and the Apple® (HR: R² = 0.99 and SC: R² = 0.87) and Fitbit® (HR: R² = 0.72 and SC: R² = 0.83) devices. 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. Linear regression analysis demonstrating the association between each wearable device and the Apple Watch and Fit Bit Charge for recording step count and heart rate.