Hoping the wisdom of the crowds might provide some insight here:

Did a 50K ultra run in Zion this last weekend. Great event btw. Beautiful, Mild temps (45-70) and super low humidity. Not sweaty weather, rather the sweat would dry immediately and leave a salt crust on skin.

Hydrated well and did nutrition well (slight tapering at end as stomach didn't want more).

Noticed about 14 miles in that hands felt swollen. My running partner experienced the same at mile 20 and numerous runners at aid station were complaining of same. So, my health concerns were somewhat ameliorated.

Felt pretty good and heart rate was about 135 through mile 24. Then,

HR dropped to about 122 and pegged there. I just started feeling exhausted. i.e., winded, legs very tired despite downhill. Sucked it up and ran/walked up the last 5 mile up the steep hill. HR stayed in low 120s, if I were running by "feel of heart rate", I was running up to about 148 and walking it back to 125 (repeat).

Finally, this felt a lot like finishing the run at the end of an Ironman. However, the hand swelling and the low HR were new.

I'm old and slow. Just want to see if anyone thinks this is unhealthy, normal, or Apple Watch hr error.

Thanks,