Heart Rate Readings whilst using Zwift

Interesting......good to see the physical test ad outcome.

Is it safe to assume then, the watch has picked up the physical movement of you running and altered the HR sensor to ensure it's tracked?

Given i was on an indoor bike this morning, perhaps there wasn't enough upper body movement (as 4493496 suggests) for the watch to detect the increased activity?

Just a question - did you start an activity or HR broadcasting on the watch when starting to run, or was the watch in the plain smartwatch mode? If you start an activity, or HR broadcasting (can be without any activity recording, and without connecting any device), the HR sensor goes into the full power mode (solid green light) vs the low power mode (rapidly flashing).

The Epix was left on the watch face without starting an activity at any time. I used a Polar H10 chest strap paired to my Zwift laptop and my Fenix 6X for recording.

Once I'd finished the run I left the 6X recording while I synced my Epix to Garmin Connect and reviewed my heart rate graph. Here's how the graph looked before I saved the activity on my 6X.

Note that there is a Move IQ activity for the run in the timeline. Also note that the two minute max average of in line with my absolute max during the run of 184.

Here is my graph for today after syncing the recorded activity from my 6X until some time after ending the run....

There is no change in the graph other than the Move IQ notation earlier note being replaced by the recorded activity.

I checked the HR displayed on the watch face quite frequently throughout the run. Like I say, a shaky start at the beginning before it caught up and then excellent HR tracking until I stopped running and started my walking warm down. Then the watch trailer behind reality as my heart rate lowered and it completely fell apart when I stopped moving my arm.

No evidence of cadence lock at any time.

Tomorrow, I will try the same test conditions whilst indoor cycling instead of on the treadmill.

As already stated, I do this for indoor cycling and the WHR tracks the HRM just fine. The only time my arms move is if I take a drink or move from tops to hoods and vice versa, probably more than 10 mins in between movements.

That may be your experience.

I just tested this morning and, outside of activity recording, the watch OHR does not detect heart rate above 123 BPM even for a ride of over an hour with heart rate of 160+ BPM. Every time I compared Epix HR to my chest strap the Epix was typically 50-60 BPM below the chest strap. This is WITHOUT starting to record an activity on the watch, just the standard day long heart rate monitoring.

Here is my heart rate graph from the watch since midnight and including my 80 minute ride.....

Note that the indoor ride has not been detected by Move IQ. You can see the elevated heart rate, but it falls far short of reality.

Once I sync my 6X the ride activity appears in my timeline, but the graph itself does not change....

And here's my HR data as recorded from my chest strap by my 6X....

So quite clearly,  without starting an activity (or Move IQ activity detection) the Epix is completely clueless when it comes to reliably tracking highly elevated heart rate.

Here are a couple of comparisons of Epix HR from the watch face vs chest strap during the ride....

123 vs 173 after 18 minutes riding

108 vs 170 after 73 minutes riding.