Aerobic decoupling: definition, formula, example

Run for 90 minutes at an even, easy pace and your heart rate will rarely stay flat. It creeps up: a few beats in the second hour, more if it is hot, more again if long runs are new to you. Aerobic decoupling puts a number on that creep. It compares how much pace (or power) you get per heartbeat in the first half of a steady effort with the second half. Joe Friel popularised it for endurance athletes, and TrainingPeaks labels it Pa:HR for pace and Pw:HR for power.

How decoupling is calculated

First, split the steady part of the effort into two halves of equal time. For each half, compute the efficiency factor (EF): average output divided by average heart rate. Then express the loss from one half to the next as a percentage:

A positive value means you produced less per beat in the second half: heart rate rose for the same pace, or pace fell for the same heart rate. Speed should be grade adjusted when the course is not flat, otherwise a climb in the second half looks like fatigue. The usual reading, from Friel: under 5% is steady, 5 to 10% shows some drift, above 10% the effort outran your aerobic endurance.

A worked example

A runner does 90 minutes on a flat loop at 5:20 per km, which is 187.5 metres per minute.

• First 45 minutes: average 142 bpm, so EF1 = 187.5 / 142 = 1.320
• Last 45 minutes: same pace, average 150 bpm, so EF2 = 187.5 / 150 = 1.250
• Decoupling = (1.320 − 1.250) / 1.320 × 100 = 5.3%

Just over 5%: some drift. Had the second half averaged 146 bpm, the figure would be 2.7%, comfortably steady. On the bike the arithmetic is the same with watts: 200 W at 135 bpm, then 195 W at 140 bpm, gives EF values of 1.48 and 1.39 and a decoupling of 6.0%. Note that the power dropped and the heart rate rose; both count against you.

How to use it in training

Decoupling is most useful as a repeatable test of aerobic endurance during a base phase. Pick a steady session you will do again: a 75 to 120 minute easy run on the same flat loop, or a ride at the bottom of your endurance zone. Keep the intensity in the same heart rate or power zone each time and watch the figure over weeks.

• If a 90-minute run sits above 5%, the duration or the pace is beyond your current endurance. Keep the pace, or slow it slightly, and build duration.
• Once it stays under 5%, extend the run, or add a steady block at marathon pace at the end and see whether it holds.
• Before a marathon or a long trail race, a long run with marathon-pace segments that stays under 5% is a reassuring sign. A large drift is a reason to rethink the target pace, or the fuelling.

OpenAthlete computes decoupling and the efficiency factor on every running activity with grade-adjusted speed and heart rate, and on rides with power. On the activity page you can leave the warm-up out with a slider or select a range of the effort, and the card shows both halves and a status: steady, some drift or marked drift. Under 20 minutes, it flags the effort as too short to judge endurance.

Limits

Decoupling only means something on a steady effort. Intervals, stops at traffic lights or a run that ends with a fast finish break the comparison. Heat is the biggest confounder: Coyle and González-Alonso (2001) describe how rising body temperature drives heart rate up while the volume pumped per beat falls, and dehydration adds to it. A 7% decoupling on a 30 °C afternoon and 3% on a cool morning can describe the same fitness. Optical wrist sensors that lock onto cadence, a chest strap with dry contacts or GPS drift under trees all distort one half. Finally, decoupling says nothing about speed: you can be perfectly coupled at a pace that is too slow for your goal. Read it next to your pace, your training load and the conditions of the day.