Glossary
Heart rate drift
Heart rate rising while pace stays flat. Under 5% across a long run is normal. Over 10% usually points at heat, dehydration or missing endurance.
What it is
Your heart rate climbing while your pace stays the same. It happens on almost every long run, and the size of the climb is the useful part.
Compare the first half of a steady run with the second half. Divide the pace by the heart rate in each half, and see how far apart they are.
| Drift across a long run | Usual reading |
|---|---|
| under 5% | well within your endurance |
| 5-10% | at the edge of it |
| over 10% | heat, dehydration, or the run was too long |
What it is not
It is not proof you are unfit. Coyle and Gonzalez-Alonso showed drift rises with heat and fluid loss in trained athletes too.
It is also not a reason to speed up when heart rate falls late in a run. That usually means you are tiring, not recovering.
What moves it
- Heat and humidity. The largest single cause.
- Fluid loss. Blood volume falls, so the heart beats more often.
- Duration. Drift grows with time on feet.
- Your endurance base. More easy volume flattens it.
When to ignore it
Ignore it on any run under about 45 minutes. There is not enough time for the signal to appear.
Ignore it when the heart-rate trace is noisy. A wrist reading that dropped out for ten minutes cannot tell you anything about drift.
Sources
- Coyle EF, Gonzalez-Alonso J. Cardiovascular drift during prolonged exercise. Exerc Sport Sci Rev 29(2):88-92, 2001 primary
- Wingo JE, Ganio MS, Cureton KJ. Cardiovascular drift and VO2max during heat stress. Exerc Sport Sci Rev 40(2):88-94, 2012 primary
- Maunder E et al. Decoupling of internal and external workload during a marathon. Int J Sports Physiol Perform, 2021 primary