As a cyclist finishes their base training, it's important to evaluate how strong your aerobic base truly is.  By using a metric called “aerobic decoupling” you can quickly and easily determine if your base work was effective and if you're ready to progress into harder high-intensity interval training.  But before you can evaluate your aerobic fitness, you need to learn what aerobic decoupling is and how to use it effectively.
What is aerobic decoupling and why do we care about it?
If you're training with power, aerobic decoupling is a key measurement to evaluate as you perform base training.  Aerobic decoupling is a numerical measurement of aerobic efficiency and endurance.  It's representative of your body's ability to process oxygen and produce energy (as detailed in Biohacking Energy Systems) and is a marker of overall aerobic fitness.
Before you can appreciate the value of aerobic decoupling, here's a refresher about how your body produces aerobic energy and the concept of heart rate drift.
To produce energy, your body takes in oxygen through the lungs and passes it to working muscles via the blood stream.  Oxygen and a fuel (fat, protein or sugars) are processed in the mitochondria to create Adenosine Triphosphate (ATP).  ATP is the actual energy molecule that allows your muscles to contract and create pedaling force.  Constant demands on the aerobic energy system can create a phenomenon known as heart rate drift.
Click through the jump to read the show notes for episode 78 of the Tailwind Coaching Podcast:
Heart rate drift and aerobic decoupling
Heart rate drift occurs because your body is an organic system and it fatigues under use.  For example, in a 40k time trial, you will be working as close to your functional threshold power as possible.
Initially, your body will recruit as few muscle fibers as possible in order to propel your bike forward.  As your body continues to work at a constant level, more muscle fibers are needed to continue producing power.  Your body recruits fresh fibers as some of the initially used muscle fibers begin to fatigue.  The fatigued fibers can't produce as much force so your body calls on other fresh fibers to help.  The more muscle fibers that are being utilized, the more oxygen is necessary to power them.
Your body requires more blood volume to transport oxygen as it recruits more muscle fibers.  Increased cardiac output most commonly occurs via an increase in heart rate.  This creates the phenomenon of HR drift.  Combined with a constant power output, you have aerobic decoupling.
This increase in muscle fiber recruitment and increased oxygen demand creates aerobic decoupling.  By definition, aerobic decoupling is a measure of how your heart rate reacts to a given steady power output.  Typically, you'll find either an increase in HR for a given intensity or drop in power for a given HR.
*Note that aerobic decoupling is completely ruined by anaerobic efforts: measuring aerobic decoupling requires a completely aerobic effort.
What is aerobic decoupling used for?
Aerobic decoupling is a great measure of aerobic fitness.  Essentially, the longer you can ride without experiencing significant decoupling, the more fit your are.