Last Sunday I took part in Marathon race. For the first 30 mins my watch was telling my that I have something about 120bpm heart rate, while during normal workouts with the same pace I usually have something about 150bpm. Then I started to fell anxious that my watch is not working properly. I stopped workout for a 3s and started again(you can see clearly drop in pace metric at ~40 min) and then my watch suddenly started to telling me that actually I'm closer to 180bpm which is huge difference. That thing could mess up my race If I wouldn't be cautious!
Furthermore as you can see, most of the race time I was running in ~180 bpm which is my zone 5. As far as I know it is nearly impossible to run 2 hours in zone 5. Probably my zones and threshold are incorrect. OR. the zones are correct but my watch would give me incorrect heart rate during race. To be honest during race I didn't feel like I have 180 bpm.
It is worth to mention this is not the first time I get such anomalies with HR from my watch. I got the same problem couple of times during my normal workouts. Of course, first thing that I always do is to clear the sensor and try to tighten my wristband, pause and start workout and it usually doesn't help. Is my Garmin broken?