This is why I use a chest strap heart rate monitor as a cyclist. I see the difference between when my hands are down on the handlebar. It varies around 10-15 bp

This is why I use a chest strap heart rate monitor as a cyclist. I see the difference between when my hands are down on the handlebar. It varies around 10-15 bpm (the watch is lower than the chest strap in this scenario). Also, while hiking I see the difference after 2-3 hours. Hikers know the hands swell after 2-3 hours of hiking (you will see people raising both hands while running, that's normal) and this affects my watch heart rate calculations. There is always a discrepancy between the strap and wristwatch. Watch tracks if my heart rate is high or not but for intense sports like mountain biking I don't expect it to be accurate at all. I am using a Garmin Instinct 2X Solar Tactical (watch) and a Garmin heart rate chest strap.