Aerobic decoupling: what it is, how it works and why it’s important

Maintaining a constant heart rate and power output is key to unleashing your perfect race. Just look at the numbers…

Ironman athlete on the bike leg

Aerobic endurance is the most basic physical ability required to prepare for an Ironman triathlon. The next most important is muscular endurance, which is built on an aerobic endurance foundation.


Aerobic endurance fitness is the ability to maintain a steady effort below the anaerobic threshold for hours at a time with minimal fatigue. When your aerobic endurance is exceeded and fatigue sets in, one of two situations becomes readily apparent – either your heart rate begins to drift up, or your power
(on the bike) or speed (on the run) drifts down.

In fact, both your heart rate and output can be affected. But when aerobic endurance is at a competent level, both of these markers of fatigue remain relatively stable, with no upward or downward drifting for several hours.

Watts and HR

The graphs opposite illustrate examples of heart rate and power variation over a 4hr steady-state bike ride. The latter example is an athlete with good aerobic endurance, shown by the minimal deviation in heart rate and power output over the first half compared to the second.

What’s all too common for triathletes who don’t fully develop their most basic element of fitness, however, is that their power output or speed slowly descends, even though their heart rate remains relatively steady (shown in the first graph). I call this separation of power/speed and heart rate ‘decoupling’. An excessive amount of decoupling when riding or running steadily at your goal race effort is a sure sign of inadequate aerobic endurance. I consider
anything in excess of 5% decoupling to be excessive.

Work it out

Decoupling can be measured precisely for both power and pace using WKO+ software (from But you can also determine your rate of decoupling using the software that comes with your power meter, GPS or accelerometer.

Begin by finding your average heart rate and power or speed for each half of the workout. Then figure out the ratio between the two. To do this, divide the average bike power or run speed by the average heart rate for each half. Subtract the second half quotient from the first half quotient and divide the answer by the first half quotient. The result is your percentage of decoupling. Don’t be put off by how tricky this sounds – it’s easy to work out as you can see in the example below.

Be aware that your aerobic fitness is specific to a given duration. So you may remain coupled for 2hrs at race effort, but beyond that begin to decouple. This demonstrates that you have only about 120mins of race-specific aerobic fitness, which is fine if you race Olympic-distance or shorter but inadequate for Ironman distance.

Iron-distance competitors should decouple no more than 5% for four hours while riding at goal effort and for two hours of race-like running. When this is achieved your aerobic endurance is well established.

Output-to-input ratio

Another way of gauging aerobic endurance progress is to compare your long workout output (bike power or run speed) with your workout input (heart rate). Dividing output by input also tells you how you’re progressing aerobically.

It’s simple maths. Just look at your power meter to see what your average power was for a steady ride at goal pace. Divide that by your average heart rate and you have the ratio. The same can be done for running by finding your average speed and dividing it by your average heart rate.

Do this for each of your long, race-like aerobic endurance rides and runs, and note the ratios in your training log. You should notice that the ratios become higher. Of course, there will be times when the ratio goes in the wrong direction. That’s to be expected for many reasons but the trend over several weeks should be upward.

Keep tabs

It’s a good idea to monitor both of these aerobic endurance tools throughout your training sessions – regardless of what part of the season you’re in – and it’s especially important when undertaking goal-intensity targeted workouts.

As for sessions to improve your aerobic endurance, I’d recommend doing at least six long, aerobic endurance workouts in each sport, in an eight-week period prior to peaking for the race, even if you have minimal decoupling and your output-to-input ratio improves significantly sooner than that. These workouts may be part of a combined bike/run session. For the bike-focused effort, warm up for 10mins or so as you build to your long-course power or heart-rate goal. Ride for up to 4hrs. Finish with a run of about 15mins at your planned race pace.

Before a long aerobic run I often have my athletes do a 1hr ride including three 12min intervals in zone 3 with 4min recoveries. This serves as a warm-up for the 2hr run at goal race speed.

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