The heart rate reported by my 955 during runs has been quite high lately. Today I ran a half marathon, and I spent 81 min in Zone 5!

My max HR in hard 5k runs is often 177bpm, and I'm 57. Could this be normal for you? The formulae for max HR are for the "average person" and there are always statistical outliers. My resting heart rate is about 50-55bpm.

Did you use a HR strap or were you relying on the optical sensor in the watch? It is very easy for the watch's sensor to get into "cadence lock" as others have regularly reported here. Even a cheap alphabet-soup-brand HR strap from Amazon will outperform the optical sensor in most watches.

runningwritings.com/.../cadence-lock-why-gps-watches-have-hard.html

The heart rate monitor is unusable while running, my faithful truth sense broke and everything went bananas, heart rate lower by 15/20 beats and peak without motivation, the problem is that it compromises all the performance data, absolute madness for the top of the range models

Please read the last two year's posts on this forum for more details on how many people have been having problems with the OHR in these watches when the algorithm was changed from the one it originally shipped with.

A pre-scripted response might appear to be helpful but it isn't contributing anything to the discussion.

PS. Welcome to the forums. Have a read back through past topics to get a flavour of what's been happening.

With a higher than usual heart rate it Is it possible you could be ill but with no obvious other symptoms. Similarly, wearing the watch at night and getting low or unbalanced HRV readings can indicate sickness whether or not other symptoms are present.  Some sicknesses can be more or less asymptomatic.

I totally agree with you friend