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.