Paper in which Evan L Matthews is co-author: https://scholar.google.com/citations?view_op=list_works&hl=en&hl=en&user=YHSEdWwAAAAJ&sortby=pubdate https://journals.lww.com/acsm-msse/Fulltext/2017/08000/Variable_Accuracy_of_Wearable_Heart_Rate_Monitors.22.aspx and compared them to the accuracy of the 3 lead ECG device Quinton Q-tel RMS: