Even experienced distance runners, cyclists, and other endurance athletes may not know what aerobic decoupling is or why aerobic decoupling matters as an endurance athlete.
After all, unless you regularly study exercise physiology or have a background in endurance training, there’s a good chance that aerobic decoupling is not a metric or concept you have been introduced to despite the hours per week you spend logging your workouts.
So, what is aerobic decoupling, and why does it matter? How can you use aerobic decoupling to inform your training as an endurance athlete?
In this training guide, we will discuss what aerobic decoupling means, why aerobic decoupling is important to understand for runners and endurance athletes, and how to prevent aerobic decoupling with your training and workouts.
We will cover:
- What Is Aerobic Decoupling?
- Why Is Aerobic Decoupling Important?
- How Do I Know If Aerobic Decoupling Is Happening?
Let’s get started!
What Is Aerobic Decoupling?
In a nutshell, aerobic decoupling refers to the phenomenon wherein your heart rate during exercise drifts away from whatever heart rate you normally hold at the cycling power or running pace/speed you’ve been maintaining in a workout.
As can likely be surmised, aerobic decoupling occurs when the aerobic “coupling” is lost.
Endurance coaches and exercise physiologists consider the heart rate to be “coupled” when your heart rate during an aerobic workout is running parallel to either your pace or power metric when you are maintaining a steady pace or steady power.
Essentially, what we have seen play out in research studies and through countless observed metrics with athletes in training is that the heart rate during exercise is correlated to your intensity in a fairly linear manner.
This is such that we can actually use heart rate during exercise as an observable biometric to represent the relative percentage of your VO2 max or effort level that you are working at.
Once you reach an aerobic steady state at a submaximal workload (below the anaerobic threshold), your heart rate should remain relatively constant as long as you are maintaining the same running pace, cycling speed, or cycling power.
Therefore, if you make a graph that shows your exercise heartbeat on a minute-by-minute basis during a steady-state effort, the average heart rate should run parallel to the average relative percentage of your VO2 max or power that you are outputting.
This is what is meant by your heart rate and your speed, effort, or power in an endurance workout being coupled.
Aerobic decoupling occurs when the heart rate metric drifts away from this parallel relationship even though your running speed or power output stays constant.
For example, marathon runners are sometimes familiar with the concept of cardiac drift.
Cardiac drift refers to the phenomenon by which your heart rate increases even though your running speed and/or intensity level or effort level in your race or workout remains constant.
There are different reasons why cardiac drift occurs, but the most common one (at least in terms of cardiac drift during marathon running) is due to dehydration.
Cardiac drift is an example of aerobic decoupling.
Essentially, if we were to chart the heart rate while running relative to the runner’s average speed, there would come a point at which the parallel relationship where these metrics were “coupled“ deviates from one another, and the heart rate starts to increase faster than any change in running pace.
This point where the heart rate value or trend line starts to drift up and away from the parallel relationship with pace or power output is where these metrics have become decoupled, so we can say this is where aerobic decoupling has occurred.
Aerobic decoupling is often considered to occur just when the heart rate starts to drift up when you are trying to maintain a certain pace or power output in an endurance workout.
However, aerobic decoupling also occurs if you are trying to keep your heart rate within a certain target heart rate zone and have to drop your power output or slow down your running or cycling speed in order to stay within the heart rate zone.
Here, in order to stay within your target heart rate zone and not experience an increase in heart rate, you have to slow down or ease up on the intensity level.
In either scenario, the relationship between heart rate and pace/power output has decoupled because the parallel relationship has been lost.
Why Is Aerobic Decoupling Important?
In the example provided with cardiac drift in a marathon runner, plotting heart rate relative to running pace would indeed show aerobic decoupling, but generally speaking, cardiac drift is mainly caused by dehydration.
As mentioned, heart rate during steady-state aerobic exercise should be relatively constant for an endurance-trained individual.
Heart rate is one of the two components of cardiac output, the other being stroke volume.
Cardiac output refers to how much blood your heart is pumping per minute, so the equation for cardiac output is Q = HR x SV.
If your aerobic fitness and endurance are developed, cardiac output should be steady at a given submaximal workload, which means that as long as stroke volume remains constant, your heart rate at that given workload should remain constant.
The primary reason we see cardiac drift occur in a marathon or long endurance workout is because as the athlete gets dehydrated, blood plasma volume decreases so stroke volume, or the amount of blood pumped for every bit of the heart, decreases.
In order to compensate for this reduction in blood volume pumped with every beat, the heart has to beat more times per minute so that the cardiac output doesn’t consequently decrease.
Thus, we see the exercise heart rate trend upward in the latter stages of a marathon, long run, or long endurance workout if the athlete has not been able to hydrate adequately to maintain normal hydration levels.
Aerobic decoupling outside of instances of cardiac drift due to dehydration can provide insight into your aerobic fitness level and readiness to progress your training.
Theoretically, if your aerobic endurance and cardiorespiratory fitness can’t support the pace or power level that you are trying to maintain without your heart rate decoupling, it is a good indication that you need to build your aerobic endurance for that pace or power level.
For example, imagine a scenario where a runner is trying to hold an eight-minute pace for a half marathon.
The runner needs to get to a place where the effort level to sustain running eight minutes per mile is below the anaerobic threshold so that the heart rate and muscular and metabolic fatigue do not cause aerobic decoupling, exhaustion, and ultimately, the need to slow down.
You can observe how long you are able to keep the heart rate and pace metric coupled before the heart rate starts to drift upwards.
Heart rate decoupling significantly earlier than 13.1 miles will indicate that the athlete needs to develop the aerobic base more and work on building aerobic endurance at that effort level.
How Do I Know If Aerobic Decoupling Is Happening?
In order to identify aerobic decoupling in your own training, you have to use a heart rate monitor along with a fitness watch.
If you are running, you can identify aerobic decoupling from a GPS running watch that keeps track of your pace and/or running power along with your heart rate.
For cycling, it is generally best to use cycling power along with your heart rate data to detect aerobic decoupling in your workouts because power is a little more constant than speed for outdoor riding because of the significant impact of climbing and descending hills on the bike.
You can use the concept of aerobic decoupling to assess your current aerobic fitness or aerobic ability by trying to identify how long you can hold a certain running pace or cycling functional threshold power (percentage of FTP) before heart rate decoupling occurs.
Most endurance coaches recommend that runners assess aerobic ability via aerobic decoupling by trying to hold 75 to 85% of their threshold pace for as long as possible, while cyclists should ride at 65 to 75% of FTP.
Fitter or more experienced endurance athletes should err on the upper end of these ranges.
The longer your heart rate stays coupled with your power output or steady-state sub-threshold piece, the better your aerobic fitness.
Periodically reassessing how long your heart rate stays coupled to your pace/FTP, or how long you can maintain your cycling or running power or pace at a given heart rate, will help you assess whether your training program is indeed effectively improving your aerobic fitness.
You can learn more about running power as a useful metric in your training here.