I really LOVE my Apple Watches! That being said I have noticed that when I go mountain biking and/or Weight Lifting (activities that cause a lot of Arm/body movement). I go to check my heart rate and at times my BPM is at 57. I then have to take the watch off my wrist and wipe the sweat off and reattach and I am good to go for the moment. Stopping to do that kills the moment so I pulled out my POLAR 57 strap out of my drawer and attach it to my watch and the readings are rock solid. I know it is extreme but I hate to lose calories when the heart monitor is out of order. I notice it mostly when doing extreme mountain biking and I am making a lot of jerking movements. I am wondering if I am the only one? Ps. I wear my watch using my nike strap very tightly about an inch away from my wrist.