I've always had a low heart rate and I'm just an old guy (68). Low 40s at rest and even as low as 38 when I sleep according to my Apple watch. For the record, max is around 175-178 last time it was estimated. and I haven't seen anything over 172 in a long, long time.
I have averaged over 6,000 miles per year since 1982, so maybe that has something to do with it... or maybe it's just hereditary, don't know and don't care.
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Keep the chain tight!