I just noticed that the plastic strap on my sports model AppleWatch has gotten looser. I think that made my heart rate a bit less precise. I tightened it but the point is that the wrist strap can stretch over time. My question is what is the difference among the different types of heart rate measurements? What is the difference between elliptical, outdoor running, outdoor cycling, etc? Does the watch just take your heart rate at different time intervals (e.g. outdoor walking would take your rate once every 3 minutes and outdoor running would take your heart rate once every 30 seconds, for instance).