Same 8km loop I have run since March, same pace, same time of morning. In April my average heart rate was around 148. This week it is 160 to 163 for a pace that feels a bit harder but not dramatically. Sleeping fine, not ill, no change in training. It is 27C and humid in the mornings now instead of 9C. Am I losing fitness or is this the heat?
Also check hydration status before the run, not just during. Starting a summer run slightly down on fluid is worth several beats on its own, and most people are because they sweat overnight now too.
Weighing yourself before and after a run for a week is a cheap way to see how much you actually lose. Mine was over a litre an hour in July, which explained a lot.
Move the run earlier if you can. The difference between 6am and 8am in July where I live is about 6C and a whole world of sun exposure.
One caution: if the drift persists on cool mornings in September, or your resting heart rate is also up by ten, that is worth looking at separately. Heat explains a summer pattern, not a permanent one.
@dartfit_dana · 5d ago · 3 replies
Heat, and 12 beats is a completely ordinary amount for a jump from 9C to 27C with humidity on top. Your body is sending a large share of blood to the skin to shed heat, which means less returning to the heart per beat, which means more beats to move the same oxygen. Humidity makes it worse because sweat that does not evaporate does not cool you. Run by effort for the summer and accept slower paces at the same heart rate, and you will find that in September the same effort produces paces you have never seen before.
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@dartfit_dana · 4d ago
Exactly. And heat acclimation is real training in its own right. Two to three weeks of consistent running in it and you will claw back a chunk of those beats, though never all of them.
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@flakey_test · 3d ago
So the summer paces are not a step backwards, they are just being paid for differently.
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