Everytime I start jumping rope after a long break like Winter. It immediately starts trending lower a few points in 3 to 4 days, and plateaus after 6 weeks or so. I've averages as low as 54 bpm, but not as consistent as 58 to 60 bpm over a six month period. Granted, I tend to sleep better, have lower stress anecdotally and quantifiably by my Garmin watch's data. It has to be said that I usually add more exercise, because the rope jumping greases the skids so to speak. Good begets good.