I'm using the 5X for one week now and I've done 5 runs so far. The first run out everything worked fine. My HR was exactly what I was expecting for the pace I was running at (~140 5:15/km ; ~170 4:15/km). But after that run it doesn't, the heartrate displayed is way too low. My max heartrate is around 200 and when running at 5K pace or doing a tough track session it's usually around 185 avg. For some reason the maximum measured HR doesn't go any higher than 155 or so anymore. I tried putting on the watch tighter or a bit more loose with the same result.
When I'm not running everything seems fine. Rest heartrate looks good and when for example walking my dogs the heartrate also seems to be correct.
Examples:
Resting heartrate 40-45 (normal)
Walking around 90-110 (normal)
Running at easy pace (5:00/km)140 (normal)
Running at high pace (3:10-3:30/km) ~150 avg (abnormal).
All with the most recent standard firmware.
