If accurate HR is important to you, consider a monitor like the HRM-Run or the Scosche Rhythm. Otherwise, you can try some things to help like making sure the watch is snug. Most people tend to wear the band too loose. It shouldn't hurt or leave marks but it should be tight enough that you can't slide a finger under the band during workouts. Optical HR can also be especially poor during outdoor cycling activities.
Thank you guys for your answers. I have the chest strap, probably this will make a better job. I was just wondering because I have sometimes auricular fibrillation, so I thought could there be probably a kind of bradycardia (low frequency ) going on. It just looks also that this misfunction starts almost at the same time, around 15min after the start.
