You held the same pace from the first kilometre to the last. Your heart didn’t. Somewhere past the hour it began a quiet climb: two beats, then five, then eight above where the morning started, with nothing on the watch to explain it. Decoupling puts a number on that climb.
What decoupling measures
Aerobic decoupling is the drift between what a pace costs you early in a run and what the same pace costs you late, written as a percentage. On a steady effort, pace and heart rate hold a bargain: this speed for this many beats. Decoupling measures how badly the bargain breaks in the back half. It is the closest thing running has to a durability gauge: not how fast you are, but how long your aerobic system honours the deal.
The read has to be grade-adjusted. On rolling terrain, raw pace lies: a second half with more climb in it looks like fading when it’s just a hill. debrief reads the drift in grade-adjusted pace (GAP) across the run, and checks it against heart rate at a given pace late in the run versus early. Two views of the same bargain.
Reading the number
Under roughly 5% on a steady effort, the bargain held: your aerobic system carried the work to the end, and that is what durable fitness looks like. The 5% convention comes from coach Joe Friel. The number doesn’t need to be zero. It needs to be small, on a day you ran honestly.
Distinctly higher, and the cause is almost always on a short list: an aerobic base not yet ready for the distance, a first half run too fast, heat, or fuel running out. The run itself usually says which. Even splits with high drift on a cool day point at the base. A hot first 10 k points at pacing. A dew point of 19 °C points at the air. Ninety minutes on water alone points at the fuel.
When the number is meaningless
Under about twenty minutes of steady running there is nothing to drift from; the number is noise wearing a percent sign. And on intervals it means nothing at all: surges and recoveries shred the pace-to-heart-rate line the metric depends on. A decoupling figure quoted from a fartlek is a coin flip, whoever’s app it came from.
debrief simply doesn’t score those runs. A number that shouldn’t exist is worse than a blank.
Heat skews it
A muggy day adds drift that no amount of training explains, and dew point drives it more than temperature. That’s the trap in tracking decoupling through a summer: the number climbs, and it reads like fitness leaving. debrief models your personal heat response from your own runs, so the drift the air explains is set aside before any judgment about you.
Track it on your long runs
The metric pays you back for running steadily: long runs at an even effort, first halves you’d be willing to defend. Do that most weeks, and the trend becomes worth more than any single reading. Drift falling toward 5% while the training load climbs is the quiet proof a base is being built.
debrief computes it from the quality-flagged, second-by-second record of every eligible run, grade-adjusted and weather in hand, and tells you in a sentence which cause it believes. That reading is one part of what an AI running coach should do with your data.