<i>[1] Unless the Garmin HRM is having one of its moments where e.g. it claims I&#x27;m sustaining 250bpm for 4 hours.</i><p>Are you sure you don&#x27;t have an arrhythmia? My HRM used to do that; turns out I was experiencing supraventricular tachycardia (SVT).  I didn&#x27;t have any symptoms other than my left atrium going bonkers due to a rogue electrical signal in my heart (no lightheadness, no pain, no noticeable loss of athletic performance).