How decoupling is calculated
Joe Friel's method, built into TrainingPeaks as Pw:HR for power and Pa:HR for pace, splits a steady aerobic session into two equal halves. For each half you divide average output by average heart rate. Friel calls that ratio the efficiency factor (EF). Decoupling is how much EF fell in the second half, as a percentage of the first.
If heart rate drifts up while power holds, or power fades while heart rate holds, EF falls and decoupling is positive. A negative number means the second half was more efficient, which is common when the first half includes a slow warm-up.
Why pace has to become speed
Pace is time per distance, so a bigger number means slower. Dividing pace by heart rate would make a fading runner look more efficient. The calculator converts pace to speed (3600 / seconds per km gives km per hour) before dividing by heart rate. For example, 5:00/km at 145 bpm followed by 5:15/km at 145 bpm is 4.8 % decoupling.
For hilly runs, use normalized graded pace for each half if your watch or software provides it. Raw pace on a course that climbs in one half and descends in the other will distort the result.
Reading your number
Friel's benchmark is under 5 % for a steady aerobic session: your aerobic endurance is sound at that intensity and duration. TrainingPeaks adds that 5 to 10 % points to moderate endurance limits or fatigue, and over 10 % suggests the effort sat above your aerobic threshold or you lack the endurance to hold it.
Use a session of at least an hour in zone 2, steady from start to finish, and compare like with like: similar duration, terrain and weather. Heat alone raises heart rate and will inflate decoupling. Tracking the same test every few weeks shows whether your base training is working.
How it works
- 1Ride or run steady. Do at least an hour at an even aerobic effort, ideally on flat terrain or an indoor trainer, after a warm-up.
- 2Split the session in half. In your watch app or analysis software, read average power or pace and average heart rate for each half.
- 3Enter both halves. Choose power or pace, then enter output and heart rate for the first and second half.
- 4Read the decoupling. Under 5 % is Friel's mark of sound aerobic endurance. Retest with the same session every few weeks.
Frequently asked questions
What is a good aerobic decoupling percentage?
Under 5 % on a steady aerobic session of an hour or more, following Joe Friel. 5 to 10 % suggests moderate limits or fatigue, and over 10 % suggests the effort was above your aerobic threshold.
Is Pa:HR the same as cardiac drift?
They measure the same thing from different angles. Cardiac drift is heart rate rising at a fixed output. Pa:HR and Pw:HR fold that together with any drop in pace or power, so they also catch fading output at a fixed heart rate.
Why is my decoupling negative?
Your second half was more efficient than your first. That usually means the first half included a warm-up, a climb or a headwind. Start the analysis after the warm-up and use steady terrain.
How long should a decoupling test be?
At least an hour of steady aerobic work, and longer for athletes training for long events. The test only tells you about endurance at the duration you tested.
Sources
- Friel, J. Aerobic endurance and decoupling. TrainingPeaks.
- TrainingPeaks. Efficiency factor and decoupling.
- Friel, J. The Cyclist's Training Bible, 5th ed. VeloPress, 2018.
Last updated 6 September 2026.