It’s famously well known that Lance Armstrong had a resting heart rate of 32 beats per minute. Miguel Indurian’s was reported as low as 28bpm (and he won five more Tours de France than Lance).
For the most part, your “natural” untrained resting heart rate is genetic. Some people have a naturally very low RHR while for others it’s higher. It’s the difference you make to it through endurance training that counts.
I’ve never had a particularly low resting hear rate, with my little “untrained” hamster heart beating at around 62-64bpm before I started cycling more seriously. Recently I’ve started measuring it again using a FitBit Blaze watch that I got suckered into buying Duty Free at Heathrow.
Although I was never overweight or overly unhealthy, I’m pretty much as fit as I’ve ever been in my entire life endurance wise thanks to slowly increasing the time and intensity of my cycling, although I need to step it up again very soon. Now my RHR sits around the 56-58bpm mark.
Oh my max measured heart rate while out cycling has been 196bpm, must have been a steep hill that one!