My last run on the treadmill the day before yesterday was particular bad regarding the Apple Watch’s ability to catch my heart rate. Only three little dots in the beginning of my run. So I finished my half an hour of running with an average heart rate of 107 according to the Apple Watch…
One of the reasons I bought the Ultra was that I thought that it maybe improves the Apple Watch’s ability of reading my heart rate when running. It does not. It performs exactly the same as does the regular Apple Watch. Which should not be a surprise, but … at least it was a nice excuse to go for the Ultra . 
Be it as it may, I looked into getting a chest strap to get reliable data. While checking out several options online, I stumbled upon this little fellow in some reviews as a comfortable and easy to use alternative to chest straps and I bought it for about 60 Euro:
https://www.polar.com/us-en/products/accessories/polar-verity-sense/
For sure, the product name Polar Verity Sense is not intended as a pun against “smart watches”, but it made me smile. And guess what: I paired it with the Apple Watch Ultra, put the Verity Sense on my upper arm and started a new run this morning. The Polar Verity Sense did not miss a beat. It was a joy to watch the constant heart rate showing up on the Apple Watch while running. It is easy to handle, very comfortable and apparently it just works. I am glad that I found this thing.
I am happy for those who are able to get reliable heart rate readings from the Apple Watch while exercising, but there are also quite a lot of persons who are not so lucky - also in this community, as I rediscovered: