I'm not sure I believe "max heart rate" limits. They must vary per person, perhaps significantly. In my early 40s, I did a lot of higher altitude mountain bik

I'm not sure I believe "max heart rate" limits. They must vary per person, perhaps significantly. In my early 40s, I did a lot of higher altitude mountain biking (single track, usually involving a good mix of climbs and descents). I wore a heart rate strap, which should be pretty accurate at measuring HR. According to my Garmin data uploaded to Strava, one of my long rides (4 hours moving) had an average hr of 161 bpm and a max of 192. It also shows that 15% of my time was > 174 bpm. According to heart.org, my max should have been 175. Since that's more than half an hour operating above the supposed max HR, I don't believe those max limits. Even so, I think pushing oneself to the limit for periods of time is great as long as it doesn't kill you. And now for those of us who have been sedentary for some extended time (blame COVID), HIIT can mean going up and down the stairs a few times! Perhaps I should wear my heart strap around the house; I feel like my current max HR is probably significantly below what it should be. On an unrelated note, a couple of years ago I went from not exercising to trying to learn to run. For one month, several days a week, I did a run/walk of a mere 6km. I did push myself to decrease the total time spent, and there was some progress. Before that month, my resting HR was around 65 bpm. After that month, my resting HR was in the low 50s, occasionally dipping into the high 40s. This was a bit scary to read, since online info about low resting hr supposedly means something bad; but I didn't feel bad. And after I stopped exercising, my resting rate went back to the usual mid 60s.