As an athlete, one of the things I love about my Apple Watch (Series 3) is the very accurate heart rate function. I find it dead on in every mode I have tried - except one - the walking function of the exercise mode. I hated the three FitBit watches and the two Garmin VivoSmart HR+ watches I have owned in that they were not even close to recording accurate heart rate during exercise. I have found this watch to be dead on during exercise except for what I am about to describe. When I am riding a stationary bike which I do daily, the heart rate always exactly matches a manual reading. I started using the walk mode under the exercise app just yesterday and the watch reads incredibly high in that mode and only that mode. I have tried it numerous times, and it jumps right to the 120s and typically gets into the 130s when my actual heart rate is no more than around 80. As soon as I stop that app, the heart rate reading again becomes accurate as it is for everything else I do. Does anyone have an idea why this device which is so accurate during every activity (including weight training) suddenly becomes so inaccurate when used in the Exercise walk mode?