Running

I've always figured it had to have a genetic element.

I was sedentary most of my life until I took up running just before I turned 40. It turns out despite not having a fitness or running background, running came easily to me, and I got fast really quickly, and my RHR dropped in to the low high 30s/low 40s. I trained hard, but comparable to other dedicated non pro athletes.

I felt like I was getting "more bang for my buck" with training than a lot of my peers, and I figured that had to be genetic.

Even when I stopped running over the covid years, and put on weight and lost my fitness, my RHR still stayed in the mid 40s.

These days (just over 10 years later) I'm "BMI obese", but running around 15km a week, and my RHR is back down in the high 30s and low 40s. And there is no way that could have happened just due to my training levels.