Aerobic decoupling : cardiac drift over long efforts

Heart rate increase at steady pace during a long effort, signaling cardiovascular fatigue, heat, or dehydration.

Aerobic decoupling (Pa:Hr on Garmin) measures the degradation of cardiorespiratory efficiency during long efforts. It compares the pace/HR ratio of the first half of a session to the second: decoupling <5% indicates good aerobic fitness; >8% signals the intensity was too high or heat/dehydration disrupted thermoregulation. It is an objective marker for improving aerobic base: high decoupling means the long run intensity was too high. The goal is to stay under 5% during Zone 2 long sessions.