1. In the paper from Jakub Parak, Ilkka Korhonen
Department of Signal Processing
Tampere University of Technology
Tampere, Finland
in which the Firstbeat Bodyguard has been tested against a standard clinical ECG device, there where tests at rest:
https://assets.firstbeat.com/firstbeat/uploads/2015/10/white_paper_bodyguard2_final.pdf
2. There where no tests at rest in the paper in which the group of Dr. Milind Y. Desai tested the Polar H7 chest strap, an Apple watch III and other optical heart rate monitor against a Cardiac Science clinical ECG device:
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6732081/
3. There where no tests at rest in the PLOS ONE paper in which the Polar OH1 was tested against a clinical ECG device:
https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0217288$
Practicing meditation is an activity which is performed at rest.
Paper 1,2 and 3 tested heart rate monitors against clinical ECG devices.
Clinical ECG devices are generally more accurate than chest staps.
The study of Dr. Milind Y. Desai showed that the Polar H7 chest strap had the greatest agreement with the ECG (rc=98), followed by the Apple Watch III (rc=96).