An older version of this Scosche Rhythm heart rate monitor: https://a.co/d/i4NmUeH

And FITIV Pulse app.

I found the watch heart rate monitors to be too slow to catch changes which really screws you on zone running. They probably have inconsistent readings as the watch moves around. I wear this monitor on my forearm, near the elbow. Def stays put. And it's more comfortable and convenient than the chest strap style monitor.