AEROBIC DECOUPLING DEFINED
“when heart rate is held steady during extensive endurance training, output may be expected to drift downward. This parallel relationship between input (heart rate) and output (power or speed) is referred to as “coupling.” When they are no longer parallel in a workout as one variable remains steady while the other drifts the relationship is said to have “decoupled.” Friel, 2008
CARDIOVASCULAR DRIFT DEFINED
“Cardiovascular drift” (CV drift) is a phenomenon whereby some CV responses begin a continuous time-dependent change, or “drift,” after ˜ 10 min of prolonged moderate-intensity exercise (e.g., 50–75% V̇o2max) in a neutral or warm environment… characterized by a progressive decline in stroke volume (SV) and pulmonary and systemic mean arterial pressures (MAPs) and a parallel increase in heart rate (HR), whereas cardiac output is maintained nearly constant.” Coyle & González-Alonso, 2001
In ESP Tipcast 87 I discussed the irrelevance of aerobic decoupling (AD) as both a measure of fitness, as well as a training tool. In truth, I hate the term because it is not only grossly misleading and strikes me as an attempt to repackage a well-known concept in exercise physiology as a new break through in coaching. Consider the two quotes at the top of this article. The first is from Joe Friel’s original article on AD, the other is CV drift as defined in a review paper by one of the world’s pre-eminent exercise physiologists (Ed Coyle), who I would also rate as a leading expert on endurance runners and cyclists. Aerobic decoupling implies there’s some sort of “break” in the aerobic system, when in fact there’s nothing really changing with the aerobic energy system. If anything, we should call it cardiovascular decoupling, but that would sound awfully close CV drift, and not nearly as new age. In the rest of this article, I’ll put forth a few extra pieces of information to clarify what CV drift is, and why using it as a regular metric makes little sense.
Exercise Physiology 101
Here are some key aspects of exercise performance you need to know:
VO2 Max= Cardiac Output X A-VO2 diff
Cardiac Output (L/min) = HR (bpm) X SV (ml/beat)
Performance Power = Performance VO2 + Performance O2deficit X Efficiency
Performance VO2 is itself determined by lactate threshold and VO2max. So one can see how CV drift, which decreases SV and ultimately VO2max could hamper endurance performance, though this has not been specifically shown.
Correlation does not equal Causation
The evidence to date shows that temperature and hydration are both associate with CV drift, but that the underlying cause is actually increased HR, which likely reduces the filling time of the heart; the less time spent filling the heart with blood, the lower the SV will be. What causes the steady rise in HR is still up for debate, but probably relates to an increased spill-over of cathecholamines – often referred to as adrenaline. If you actually block that increase in HR with drugs (e.g., beta-blockers work well), you can often eliminate the drop in SV. Also, if you attenuate body temperature and/or dehydration, you also reduce that SV drop. But is this necessary?
We have long assumed yes, but human performance is complex, and simply limiting say dehydration to 2% or less has been shown to be unnecessary. The fact is, we simply do not know enough about CV drift to definitely say if eliminating it improves performance or training, so for anyone to choose a specific threshold, like 5%, and then start making training recommendations from CV drift is premature at best, and misleading at worst.
Likewise, I cannot see any rationale for using CV drift for interval training, unless you’re going look a each individual interval drift score, and then you’re just fishing. Why would using a block of intervals not be useful? If you consider how AD is calculated, then you will quickly realize that during recovery HR will be very high relative to your power output, exaggerating your score. Now you could look at similar workouts and compare AD, but then what you’re really looking at is HR recovery, which you could simply measure in the first 60 sec after exercise, which has been shown to be very useful for training.
Finally, looking a negative AD values as a measure of improved fitness seems like a lot of extra steps when HR alone at any sub-max power/pace already tells you the same thing. In short, I believe Joe Friel has simply created a more complex metric that tells you more or less the same data that other, simpler metrics have already given us. In my experience in coaching and research, the more complicated you make something, the less helpful it often becomes. I would learn to triage your metrics, and not worry about AD.
Is aerobic decoupling useless?
Honestly, I would say at this point it probably is most of the time. However, you could look at it after a particularly bad performance. Another possible use is testing heat acclimitization. Measure the same sub-max workout before and after to ascertain if drift decreases. Presumably, the greater the heat tolerance, the lower the AD.
For those interested in reading more on these topics, here are two excellent review papers: