I wonder what they mean by "optimal", which this article seems to be mentioning a lot. It seems that they are referring to a maximum or a minimum in the curve,

I wonder what they mean by "optimal", which this article seems to be mentioning a lot. It seems that they are referring to a maximum or a minimum in the curve, and calling it optimal. It seems to offer the hypothesis that a low resting heart rate (RHR) is good, and then runs with it. It does not explain how it arrived at that conclusion. It feels that they saw that most people at a given age have a resting heart rate of X and therefore they conclude that this is optimal. However, I wonder if people with a lower-than-average or higher-than-average have a longer life expectancy. I understand that FitBit data would not show that, as the age-at-death seems to be one of the few data points they don't have :).