Aerobic decoupling is the divergence between running pace and heart rate during a sustained aerobic run. In an ideally aerobically efficient run, heart rate stays stable relative to pace — you can maintain a given pace at the same heart rate from kilometre 5 to kilometre 30.
In practice, heart rate tends to drift upward over a long run even at constant pace, due to cardiovascular drift — increased plasma volume loss, elevated core temperature, and glycogen depletion. A small amount of drift (2–5%) over a long run is normal; high decoupling (5%+ drift) suggests the runner was working above their aerobic threshold or was not adequately fuelled.
Decoupling percentage is calculated as: (Second-half efficiency factor / First-half efficiency factor − 1) × 100%
Where efficiency factor = pace / heart rate for each half. A well-trained aerobic run should show <5% decoupling for efforts up to about 2 hours at aerobic pace.