What keeps you moving through a ridiculously hot run? — For me, it's keeping control of my breathing, which in turn keeps me calm and relaxed, and furthermore focused on the task at hand. Now, take this with a grain of salt. If conditions and/or your own internal alarms are going off, then stop or don't go out on that run/workout/etc. — Let me just state that I am not a doctor nor a running coach. I only offer my own experiences and self experiments to share and have others take away what rings and aligns with them. I do suggest the best way to find out what works best for you, and what your body is capable of, is to go out and try it responsibly. — Remember run (bike/hike/swim/etc.) smart and responsibly. :) — By the way, the Garmin Fenix 3 has been a solid watch for me. I got mine (below) back when it was only offered with the heart rate monitor band, before the one with a built-in heart rate monitor was released. So, lately I don't where the band all that often due to forgetting to grabbing it. I also got waaay to focused on heart rate when I first starting trail running. To the point where I wasn't just letting go and running, as I was too focused on what my heart rate was at any given moment. That said, where it provides a ton of other data, Garmin calls "Running Dynamics," in addition to heart rate.