I'm not a doctor but I don't think that is true for most people. One of the most fit people I know has a resting heart rate of 40-50. Mine is usually 60-70 when I'm calm - when it is 80 or more something is generally off and I need to exercise more.
The low pulse of athletes is adaptively suppressed metabolism. It is not a good thing. The body deliberately reduces resting energy expenditure as a conservation measure because it anticipates the extreme energy demands of training.
Sprinters and strength athletes do no get a suppressed heart rate. It's only endurance athletes and others who train high volume.
This is well understood by a lot of trainers. Low pulse and low body temperature in the morning is an easy way to measure over-training.
People need to get it through their heads that highly trained "fit" athletes are not very healthy, as a general rule. They tend to develop problems relatively young. Marathon running is worst of all, each marathon run showing a statistical reduction in life span.
