Aerobic Decoupling & HR Drift Calculator
Enter your first- and second-half average pace and heart rate to calculate aerobic decoupling (Pa:HR drift). Under 5% signals a well-developed aerobic base; above 8% suggests the effort was above your aerobic threshold.
Aerobic decoupling (also called Pa:HR decoupling, popularised by Joe Friel and TrainingPeaks) measures how much your heart rate drifts upward relative to pace between the first and second halves of a run. It uses the Efficiency Factor (EF): your speed divided by heart rate.
Formula: Decoupling % = (EF₁ − EF₂) / EF₁ × 100, where EF = speed / avg HR. A well-aerobically-trained runner can sustain the same pace with the same HR throughout an easy run; a decoupling value below 5% confirms this.
For accurate results, use a flat, even-effort run of at least 60 minutes at an easy pace. Split the run exactly in half by time or distance and read the average HR from your GPS watch summary.
Checking decoupling by hand requires a dedicated flat easy run, a careful read of your watch summary, and manual calculation. Most runners only do this occasionally — so a month of Zone 3 overreach goes undetected until fitness stalls or an injury hits.
Single-run decoupling is also noisy: a warm day, a small hill, or a social run that drifted too fast all inflate the number. You need a trend across many runs, not a single data point, to draw conclusions about your aerobic base.
Vertical computes aerobic decoupling automatically from every Garmin run and tracks your Pa:HR trend over time. If your easy runs start drifting above 5% Vertical flags it and adjusts your training load — before fitness stalls.
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