For almost all of my comfortable and steady pace runs on a flat course the optical heart rate sensor goes off at some point. Sometimes it bounces back to a normal level, but often not.
I have read the official tips on how to wear the watch and have tried tightening/loosening it, but nothing has helped.
I have had the watch for about 2 months and did not have problems at first, they started after about 1 month of usage. I had no such problem with a competitors watch that I used before. I understand that the optical heart rate sensor is not as accurate as a chest strap, but the screenshots added show that it my case it's almost useless.
When cycling the sensor seems to work much better and there are no sudden spikes.
