[hlnx1]

During long steady sessions, heart rate often creeps upward even though pace or power stays the same. This is called cardiac drift, and it explains why heart-rate targets become harder to hit late in a long run.

• A main driver is fluid loss and rising body temperature: blood volume available to the heart falls, so stroke volume drops and heart rate rises to compensate
• Drift is larger in heat and humidity, and when you are dehydrated
• Some coaches compare the first and second half of a steady session (pace-to-heart-rate ratio); a small drift, often quoted as under about 5%, suggests good aerobic durability
• Large drift at a modest pace can be a sign that the session was too long, too hot or that fitness is still developing

Background: the heart during exercise and hydration science.

Do you track drift in your long sessions?

[hlnx1]

Example: a steady 90-minute run at 5:30 per km starts at 140 bpm and finishes at 152 bpm. That is about 8-9% drift. If the same run in cooler weather drifts only 3-4%, the difference was largely environmental, not a loss of fitness.

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