Running in strong heat: the 3-to-5% rule and heart rate

What heat changes

For the same pace, heart rate rises by 5 to 10 beats from 25 °C, more with humidity. The body sends blood to the skin to cool down, and less is left for the muscles. Holding winter pace means running one notch of intensity higher without knowing it.

The coach's rule: from 28 °C, pace targets are lowered by 3 to 5%, and heart rate becomes the target the watch monitors. A threshold session is then run at 85-90% of max HR, whatever pace results.

Adapting the time and the route

Quality sessions early in the morning, before 8 a.m., when the temperature is lowest. Long runs before 9 a.m., with water every 20 minutes and a shaded route or a loop near a water point.

The plan does not move sessions on its own: it suggests it in the weekly review when it sees Garmin activities run in the full afternoon with an abnormally high HR.

Reading summer sessions without getting discouraged

A threshold run at 30 °C at 4:45 instead of 4:30 is not a regression: it is the same session, in different conditions. The coach compares heart rate, not the time, and says so clearly in the review.

• — Above 28 °C: paces −3 to −5%, target in HR.
• — Quality before 8 a.m., long run before 9 a.m.
• — Compare HR, not the time.

Heat is also training: after two weeks the body adapts and paces come back up. A well-managed summer preparation often gives a better autumn time than expected.

Frequently asked questions