I am an avid hiker. This year I'm just getting back into the groove of things after a two month winter break. This past Saturday I did 18 miles on the AT (Appalachian Trail) with 4500ft of elevation gain. A very serious day by any standards. Probably more than I should have done as it was only my third hike this spring.
I noticed that toward the end of the day, my heart rate would increase very quickly. I would rest on a log for 5 minutes or so, but when I started back uphill it wouldn't take more than 10 or 15 steps before it was racing again. Earlier in the day, I could go 45 minutes to an hour or so of the same uphill grade before my heart would reach 150+ BPMs. While exhausted for sure, my energy level wasn't the problem. My heart was just pounding!
My questions is: What made my body speed up the heart rate so quickly at the end of the day? Was my brain telling my heart the muscles (which were surely depleted) needed oxygen ASAP or something? I'm sure it's just a conditioning thing since I don't normally experience this, but it kind of took me by surprise that my heart would race so easily after a break.