Cardiac drift: the number that says whether your long run was too fast
Cardiac drift is the rise in heart rate at constant pace over the course of a run. Under 5%, the pace was easy; above 10%, it was not, or hydration was lacking. Your AI coach calculates it on every long run from Garmin data. Here is how it reads it and what it does with it.
Last updated: 2026-09-19
The calculation
The coach splits the run into two equal halves, excluding the warm-up, and compares the heart-rate-to-pace ratio of each. If pace is constant and average HR goes from 140 to 152, drift is 8.6%.
The calculation only makes sense on a run of at least 60 minutes at a steady pace on flat ground. The coach does not do it on intervals or a hilly run.
What the number says
Under 5%: the pace was in easy endurance and hydration was adequate. Between 5 and 10%: pace a little high or a hot day, no consequence. Above 10%: pace too fast for a long run, or dehydration, or accumulated fatigue.
A drift that falls from run to run at equal pace is the most reliable sign that endurance is improving. It is what the coach shows you in the review when it says the base is building.
What the coach does with it
Drift above 10% two long runs in a row: the next long run is sent with a lower HR target, or the pace is lowered by 5%. It also asks you how you drank.
- Under 5%: easy. 5 to 10%: acceptable. Above 10%: too fast or dehydrated.
- Calculated on runs of 60 min or more, on the flat.
- Two runs above 10%: target lowered.
It changes nothing on a single run: an isolated drift can come from heat, sleep or a meal. It is the series that counts.
Frequently asked questions
Where can I see my cardiac drift?
In the activity analysis of each long run, on the website and through conversation. The coach cites it in the weekly review when it changes something.
Is drift reliable with the wrist optical sensor?
Enough for a trend over several runs. For a precise reading, a chest strap is better.
Can drift be reduced other than by slowing down?
Yes: drinking 150 to 200 ml every 20 minutes and running at cool hours are often enough to bring it under 5%.
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