Concept
Heart-rate drift
A gradual rise in heart rate during steady exercise even when pace or power remains similar.
Why heart rate rises
Heat, dehydration, fatigue, and prolonged exercise can require the heart to work harder even when external pace stays stable.
What it can reveal
Large repeated drift during comparable easy runs may suggest the effort is too high, conditions are difficult, or aerobic durability still needs time.
How to interpret it
Compare similar routes and conditions. One unusually hot or stressful day should not drive a training decision.
How to use it
On a steady easy run, compare the first and second halves while also noting heat, hydration, effort, and recovery.
Common misconception
Heart-rate drift is not automatically a fitness failure and cannot be interpreted without context.