To become a successful cyclist it is important to have a good level of base fitness. This is basically your body ability to process oxygen and produce energy.
What is aerobic decoupling?
Aerobic decoupling is simply a numerical measurement of aerobic efficiency and fitness. The longer you can ride without experiencing a significant decoupling the more fit you are. It is important to monitor this over the base period so that you can be sure that you have reached peak base fitness before moving onto the build phase
To test this you simply want to ride at a steady-state power target within zone 2 and record both power and heart rate. The length of the test should mirror the duration of your event within reason.
If you are using software sure as training peaks then the figure you need to look for is Pwr: Hr. As a rough guide, you are looking for a figure of less than 5% to show good aerobic fitness, the lower the better. It is important to look at a section of steady-state work and be aware that poor hydration/coffee and sleep can all affect the outcome so needs to be limited.
Heart rate drifts overtime during a workout as the muscles fatigue however this can be limited during a good base fitness block.
What does it look like?
The image below was taken from a 3 hour endurance ride. This particular athlete had a Pw:Hr figure of 7.14%. As you can see the heart rate started to drift at 1hour 20min.
I have put a trend line in the image below to highlight this.
As you can see it doesn’t matter how high your FTP figure is as on a long event it is your aerobic fitness that limits you not your power.
What can I do to improve it?
This not a quick process but basically lots of base training will help build this. I would generally work just beyond the current level and look to push it out. Regular rides will be more beneficial than one long ride once a week. You can fast track the process slightly but doing some additional lower cadence sweet spot training.