Cardiac drift on longer workouts (Aerobic Decoupling)

Hi there,

I have thought about cardiac drift recently. From what I understood there seem to be many different reasons for cardiac drift to occur:

  1. Heat. The body has to work increasing amounts to cool itself.
  2. Dehydration. The blood volume decreases the heart has to pump more.
  3. Increased fatigue. The body cycles though different muscle fibres.
  4. Mechanical reasons e.g. low inertia trainers (not sure about this one though).

What ever the reason, the body has to gradually produce more input (heart rate) to generate the same output (power) over the course of a workout.

I experimented with longer sweet spot workouts like Whiteside (5x20m of SST) this year and observed quite an increase of my heart rate during the workout. I managed to keep the power on target the whole time. The first interval finishes at 155 bpm. At the end of last interval then my HR crept way up close to 170 bpm. This puts me at 94% of max heart rate at the end of the workout, which is not anywhere close to the sweet spot zone.

According to this article where Coach Trevor Connor states:

Suppose you did a four-hour ride at a steady 160 watts. You might start the ride holding 145 beats per minute, but hours later your heart might be pumping 20 beats faster. This is known as cardiac drift. If you ride by power alone, you may be in the right physiological range initially but not at the end. Sticking to a heart rate range will keep you at the right intensity the entire ride.

Now I am slightly confused. Am I training in the right zone for the duration of Whiteside? Do I need to tone it down a little? Shouldn’t the heart rate curve be kind of flat when doing only sub threshold work? I know the general recommendation on the TR podcast is to ignore heart rate, however I am questioning if I am “doing it right” by just muscling through these longer workouts.

And a second thought: Have you ever witnessed different cardiac drift patterns of different indoor training setups? I strongly feel that pedalling inertia plays a big part here too.

Thanks for sharing your thoughts on this.

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Kind of a complex topic.

Related reading:


This is a difficult question, especially for long sweet spot work. For Whiteside, did the RPE feel like sweet spot?

94% of max is pretty high and seems above LTHR, but if you can get through the workout at the prescribed intensity, I wouldn’t make any adjustments based on a high HR.


From an RPE standpoint, the first 3 intervalls are certainly sweet spotish. The last 5 minutes of the 4th and 5th interval are usually just suvival… keeping the watts and not falling of the bike :slight_smile:

The idea behind aerobic decoupling is one of several indicators of your aerobic fitness. I use TrainingPeaks to easily measure this, and looking back almost a year ago when I finished sweet spot base 1 (high volume) all of the longer intervals like 3x20 or 5x15 ended up with 3% decoupling on the last intervals. That means my heart rate was relatively flat across the interval (ignoring the initial ramp when interval starts).

Aerobic decoupling under 5 or 6% is considered you have good aerobic fitness level.


Thanks for the links. I feel like quantifing the aerobic decouling for a certain type of workout (e.g. sweet sport) would be a good place to start. I have seen the number show up in Golden Cheetah, yet I had trouble getting much sense from it and I do not use GC much. I will give it a second look. Learning about the exact formula for the percentage value certainly helps :+1: Thanks!

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Thanks for the hints. As stated above I will look through my workout history and try to produce the numbers for my last couple of seasons.

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In Golden Cheetah I’ve only seen Pwr:HR (aerobic coupling) for the entire ride, not for specific intervals.

TrainingPeaks (paid version) makes it easy to do for any interval you select:

Here is a 3.69% decoupling on the last 12 minute interval in Mount Field. This was a workout from two months ago.

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Regarding adjusting the training intensity the article by Joe Friel contains the follwing (in case somebody was wondering about the same issue):

In the Base period it’s generally better to maintain a steady heart rate while for the Build period when you are maintaining AeT fitness keep power or speed steady.

Interesting stance. That would mean that during the winter time one might consider adjusting workout intensity based on HR.

Did eclipse yesterday and by working at no more than 100 cadence and ave of around 93 for the 20min blocks - I kept my HR much more level than I would have done if I had spun up towards 105-110.

However my legs are hurting more today.

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my interpretation: if you follow Friel’s plan he has you focus on a) aerobic endurance during base, by keeping HR in low-intensity zone, and b) on muscular endurance during build, by keeping power steady (steady state intervals).

I’m relatively new cyclist, and have seen good results - both aerobic and muscle endurance - from sweet spot base in both mid and high volume variants.

Yes. That’s probably the key here. The recommendation is to be seen in the context of Friel’s plans.

For TR then maybe the question kind of narrows down to: How deep should we go in sweet spot workouts and even more specifically: How deep is too deep for sweet spot?

Not sure I understand your question. I’ve found that sweet spot workouts during week 1 and 2 can be tough to complete because of muscle endurance. So I backpedal or tweak ftp.

For longer intervals without a break or back pedaling, its interesting to see aerobic decoupling decrease from week1 to week5 of SSB plan.

I guess another way to put it is how to answer a couple questions:

  1. How do you know when muscle endurance increases over a 6 week training block? At the beginning of the block the simple answer is you can get thru sweet spot intervals without back pedaling or taking a break. Later in the block the answer is you can do longer intervals.

  2. How do you know when aerobic endurance increases? The simple answer is watching your Pwr:HR ratio (“aerobic decoupling”) decrease over the 6 week training block. Joe Friel also adds the “when decoupling drops below 6% you are done” with improving aerobic fitness in early base (low intensity).

Hope that helps. First thing to understand is that in TR’s Sweet Spot Base plans, you are developing aerobic fitness, muscle endurance, and pedaling economy. And the answers to the two questions above are how you can track improvements in aerobic fitness and muscle endurance.

Finally, back to your “how deep” question – if you are struggling to complete efforts then perhaps your ftp is set too high and you should adjust down. Or keep ftp the same, and select easier variants of the workouts giving you the most problems completing. Or maybe you were having a bad day.

I think the kind of workout you’re talking about is not really like the kind of workout Trevor is talking about.

Whiteside sounds HARD, sweetspot or not. 5x20 minutes at 90% is a difficult workout in anyone’s money. Perhaps going to 95% max HR is a little high and might indicate that the room was a bit hot, or your FTP is a bit optimistic, or you were fatigued, but I wouldn’t consider it huge cause for worry.

Trevor, on the other hand, is talking about long (4 hour) Z2 base / endurance rides - where you do want to keeping that constant heart rate.

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FWIW, Trevor Connor is more a fan of a polarized approach, and kinda ignores sst as a method, so his comments need to be viewed in that context.

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I like this explanation model a lot:
Fatigue in endurance specialized muscle fibers leads to more muscle fiber recruitement, to keep doing the same work. A larger number / quantity of working muscle fiber needs more oxygen hence more load on cardiovascular system.

In other words, if your goal is to create a stimulus for muscle endurance adaptation, you want to see that heart rate drift.



After reading part of this thread I thought I would read up on cardiovascular physiology & just understand the basic process by which exercise induces increased heart rate.


There is no answer. Even for the most basic case. Just some reasonable ideas.

If anybody thinks they have a handle on this, do tell. By what mechanism does a muscle under stress cause the heart to beat faster?

I was suffering from cardiac drift on Z2 rides doing high volume traditional base. Hr started stable at 130, at the end of 4 hours hr was at 170+. What I found, in my case this was caused by lack of cooling and then dehydration. The drift is pretty noticeable because at the same output the heart rate and perceived effort is high but breathing isn’t.

I was using 2 20" house fans, exercising outside at temps of 45f-50f (but humid because of rain), had 3’ diameter puddles of sweat under me. That week I got a much more effective 24" commercial fan and hr drift was immediately much more under control. Hr still drifts but now its to 145-150 instead of 170 and perceived effort is much more under control.

I believe current science says cardiac drift is caused by overheating, dehydration and low electrolytes - the body is under stress but indirectly from the work you are doing.