I wear my 4s while on a bike attached to my Tacx Flux2. At the same time, I have a Garmin HRM chest strap on that's connected via bluetooth to the Tacx desktop software running on my laptop. The HRM heart rate Tacx desktop app while training will consistently start out at approx 70 bpm and will run as high as 140. This is consistent with readings that are registered when I'm using my Garmin 830 with the same HRM chest strp. My heart rate is controlled by a implanted pacemaker. My rest rate is 70 and max is 140. I am 100% paced.
While all this is happening, my Vivoactive 4s is registering between 56-70 on the low end and a maximum of less than 100, often no higher than 72-80 bpm. Is there an explanation for this other than the design accuracy of the 4s?