Can someone give me some pointers about what's going on in this graph. It's a trend that I've seen a lot over the year but I've never really tried to work out what's going on until now. The reason I ask is because I've always ran up to half marathons without any water or gels etc and I've been fine, so when I'm running and my heart rate is constantly increasing throughout the run and never plateauing It hasn't been a problem. Now I want to push the distances more and the increase in heart rate is now leading me to get incredibly close to my max heart rate (2-3bpm lower than MHR by the end), whilst at the same time my pace is either pretty constant, or even dropping. Clearly this indicates that I'm very unlikely to go much further than the 27km you can see in this graph. Indeed at the end of this run (my longest ever) I was having to repeatedly stop in the last ~4km as I just couldn't keep going. I was completely spent by the time I finished.
What causes this endless increase in HR? Is it that I'm getting dehydrated and my heart is having to work harder to push, I assume, slightly more viscous blood around to meet the same demand for fuel/oxygen? is it my body burning through my reserves of glycogen? something else?
Any ideas on how to get on top of this and stop the HR constantly rising like this? I guess I'd expected one's HR to flatten out and remain more or less at a constant level but that's not what I see. I should add that this graph is just one run and a longer one that normal for me, but I see the same pattern in most runs.
For info, I'm 44, 95kg with a max heart rate of probably about 185 and I'm running something like 50 miles per week and I do at least one half marathon every month, and I've been running for about 20 years, so this isn't a new sport for me.