Why does heart rate go up throughout a run even though you keep the same pace?

Question

I’ve noticed that my heart rate goes up towards the end of a race, even though I’m running at the same pace throughout the entire race – why is that?

E.g. I had a run at a pace of 5:50 min/km, and at the end of the run I had to run at 7:00 min/km to keep the same heart rate.

In: Biology

Answer 1

Your body becomes less efficient as it fatigues so your heart has to work harder to maintain the same effort.