From the Coach guide
How much more heart rate the same pace cost later in a long run.
Coach compares your heart rate with your pace in the first and second halves of a run. A small rise is normal on long runs.
Coach leaves out the first 10 min and any stops. It needs 30 min of steady running, so most readings come from long runs.
Less than 2.5% reads as steady. Studies use this line, but they call it arbitrary. It is not a target, and it does not change your plan.
Heat, hills, water, food and the watch's heart rate sensor all change it. Compare runs in similar conditions.
Steady pace: your pace can stay even while your heart rate rises. Apple's Cardio Fitness: that estimates VO₂ max, not one run. Other apps call this aerobic decoupling or Pa:Hr.
Exercise physiology; interview with the researcher
At a steady pace, heart rate rises slowly as the heart pumps less blood with each beat. It starts after about 10 min, and heat adds to it.
This explains why drift happens. It does not say how much drift is good or bad for one runner.
Observational study; 82,303 marathon runners
Runners whose heart rate drifted less finished faster. The study counts drift as started once it stays above 2.5%.
These were races, not easy training runs, and heat was not accounted for. The authors call the 2.5% line arbitrary.
Methods review
To track one runner over time, compare runs of similar length, in similar conditions, with the same food and drink.
The review says it is not yet clear how much change in drift means a real change in endurance.
Coaching practice, not a study
Coaches use decoupling (Pa:Hr) to see whether a runner can hold an easy pace for longer. Many use 5% as their line.
The 5% line is a coaching habit with no study behind it. Coach uses the 2.5% line from research instead.
Coach