@abnv @running In any case, getting a proper external HR sensor changes everything. I can recommend:

Garmin HRM-Pro Plus (chest)
https://www.garmin.com/en-US/p/770963

Polar H10 (chest)
https://www.polar.com/en/sensors/h10-heart-rate-sensor

Polar Verity Sense (arm)
https://www.polar.com/en/products/accessories/polar-verity-sense

@abnv @running If you’re using a chest strap with it, the values are likely correct. If you’re using the built-in wrist sensor, you’re probably experiencing cadence lock. It’s a phenomenon of optical HR sensors that pick up on your cadence instead of your HR. Compare your cadence values to your HR. If they are similar, that’s what’s happening.

In any case, best go see a doctor and perform an ECG and lactate threshold test under stress. It’s what I did, to make sure.