@Brad_Olwin I’ve been testing this by changing my watch to display pace rather than HR zones, and forcing myself to run in a focused zone. Last week, I ran tempo pace for 8 miles, and my HR drifted into Z4 for a large part, marking the run Anaerobic - no issue here, this would be my experience when running at this pace (around 7:10 p/m - my half marathon pace). All other runs have been at Z2 pace, and have stayed in the Aerobic zones in Impact.