Run analysis
Aerobic decoupling
Whether your heart rate stayed coupled to your pace, or drifted as fatigue set in.
A coaching heuristic (Friel). The cardiac drift it measures is real physiology, but the 5% cutoff is a rule of thumb, not a validated threshold, and heat or dehydration can inflate it.
Aerobic decoupling asks one question about a steady run: did your heart rate drift upward relative to your pace as the run went on? If you held the same pace for the same heart rate from start to finish, your effort stayed efficient and your decoupling is low. If your heart rate crept up while your pace held (or your pace slipped while your heart rate held), the two have decoupled, and that gap is what we measure.
Low decoupling is a sign of strong aerobic durability: you kept your efficiency as fatigue set in. It is a coaching heuristic, popularised by Joe Friel, not a peer-reviewed measurement, and the sections below are careful to say where the numbers come from.
The idea in one line
Split a steady run into its first and second half. Measure how efficient you were in each half, then compare. If the second half was meaningfully less efficient than the first, your heart rate drifted, and the run decoupled.
Step 1: efficiency per half
For each half we compute an efficiency factor: the speed you held for each beat of your heart. We use speed in metres per second, so that going faster at the same heart rate reads as a higher, better number.
EF = speed ÷ average heart rateSpeed is in metres per second and heart rate in beats per minute, so EF carries the units of metres per second per bpm. The absolute value does not matter here; only the change from the first half to the second does.
Step 2: compare the halves
Decoupling is the drop in efficiency from the first half to the second, expressed as a percentage of the first half:
Decoupling% = (EFfirst − EFsecond) ÷ EFfirst × 100Friel's guideline is that under 5% reflects good aerobic durability. It is important to be clear that this 5% cutoff is a coaching rule of thumb: it has no peer-reviewed derivation, and the exact line is a convention rather than a measured threshold.
Worked example
A steady run where the first half averaged an efficiency factor of 0.0180 m/s per bpm, and the second half averaged 0.0162 m/s per bpm:
- Change in efficiency: 0.0180 − 0.0162 = 0.0018 m/s per bpm
- As a fraction of the first half: 0.0018 ÷ 0.0180 = 0.10
- Decoupling: (0.0180 − 0.0162) ÷ 0.0180 × 100 = 10.0%
At 10.0% this run sits well above the 5% line. The second half cost more heart rate for the same pace, a sign that heart rate drifted up as fatigue (or heat) accumulated.
When it means something
Decoupling only reads cleanly on a steady, single-intensity aerobic effort of at least about 20 minutes. On intervals the average pace is dragged down by the recoveries while heart rate stays high from the reps, so the two halves describe different workouts and the number is meaningless. Variable terrain has the same effect: a hilly second half will lift your heart rate for reasons that have nothing to do with durability.
Honest caveats
This is a coaching heuristic, not a validated measurement, and it is easy to fool. It is heavily confounded by heat, dehydration, and caffeine: a hot run can read well above 5% with no change whatsoever in your fitness, purely because heart rate rises to shed heat. The underlying cardiac drift (heart rate creeping up over a long steady effort) is real physiology, but the 5% threshold placed on top of it is not validated by research.
The 50/50 split is arbitrary too: nothing physical happens at the exact midpoint, and a different split would give a different number. Running is also noisier than steady-state cycling for this purpose, since pace wanders with terrain, footing, and stride in a way that a fixed effort does not. Read decoupling as a soft, directional signal on comparable runs, not as a precise score. Evidence tier: coaching heuristic.