Differences in cardiac/HR response - aerobic vs anaerobic contribution

I recall @trevor mentioning on a podcast that the differences in HR response that exist between is due to being more or less anaerobic vs aerobic contribution? I was searching for this here as well as scientific reference to better understand it but being such a specific phrase, not coming up with much. Any help?

Basically I’ve seen some athletes I coach do FTP/LT efforts by power and their HR barely climb into the respective by the end of relatively long intervals (15-20min) after 2 or 3 reps. Their RPE and subjective feedback is “RPE matches Power not the relatively low HR response”. And I’ve seen just the opposite for others where HR climbs very quickly and they can sustain it and even surpass the HR zone just to keep the Power in zone. Say HR creeps into Z5 for a Z4 power effort. Where as 1st example athlete has a Z2/Z3 drift for the Z4 power. Thanks

The imbalance of HR and Power can be a result of:

  • specific training in either aerobic or anaerobic domains.
  • modality (applying HR zones of a different sport to the current sport, which requires other muscles)
  • comparing the results to an inaccurate /faulty test.
  • being fatigued / extremely rested
  • inaccurate RPE classification (in the case you describe)

Hi @dimatheny,

Sorry for the short response - running to a meeting - but wanted to make sure I gave you some sort of any answer.

I actually listened in on a symposium last Thursday (on Thanksgiving) that Dr Seiler participated in and he talked for a while about this. That slow rise in heart rate relative to power that you see in longer threshold-style intervals is called the slow-component of VO2. He discussed the slow component and how the mix of aerobic and anaerobic power can contribute to it. I don’t know if the recording is available, but here’s the link: Webinar Registration - Zoom

Very short and simplified version, when we do efforts - even efforts that are considered aerobic in nature - we are always using a mix of aerobic and anaerobic metabolism. If one athlete is more reliant on the anaerobic contribution, you’re going to see more muscle fiber fatigue and a greater recruitment of additional fibers to maintain the power. As a result, their oxygen demand is going to increase more rapidly than an athlete who is getting a much greater contribution for aerobic metabolism.

There’s actually a fair amount of research on the slow component of VO2. Very interesting stuff!

Hope that helps!

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Thanks @trevor and familiar with the VO2 slow component. It’s just odd because having physiological lab data on quite a few athletes and me personally to compare numbers to and this is exaggerated in some athletes where as not in others that I’d expect based off of some power profiling, their training history, etc.

So basically like I thought I’d heard and understood was;

  1. an athlete more anaerobically dominant athlete will see a more rapid HR response to “strong” endurance work
  2. a more aerobically trained athlete will see a slower HR response

Then the 2 questions arise; do those that fall into category #1 need to be more conservative with the power Rx so they don’t overly saturate the cardiac response? And do those #2 need to find a way to get that cardiac output up sooner (maybe with hard start intervals) so they train that spot of cardiac demand?


I seem to remember Sebastian Weber basically agree with the premise of your question. I think the specifics where with regard to two cyclists with the same FTP but different fat/carb utilization at threshold needing to train at different endurance wattages. The cyclist with the higher carb utilization needing to train at lower power. Unfortunately, I don’t remember which podcast I heard it on.

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These are great posts…the answers lead to more individual ways to do the same workout.

If the person with the better fat utilization (slower heart rate response) tries to push their heart rate up in the interval sooner, the will push too much power and likely lead to overtraining.

The opposite would ring true for the other athlete.

This is a great example of looking at limiters / strengths / weaknesses when trying to prescribe a workout for a certain energy domain.

Great thread!


Thanks @Steveneal and @schils. I had sat in on a couple of Sebastian’s in-person and online sessions and kinda caught the same thing.

But @steveneal I can agree but also play devils advocate on the take-aways you mentioned. What about the ANS response where it seems like the athlete with higher aerobic conditioning rarely elevates HR to what would seem like LT1 thus not really stimulating the SNS. Kinda a golden unicorn kinda thing getting threshold training without the SNS saturation.

While the other more anaerobic athlete is always saturating the SNS response. I could see this lead to overtraining in the latter but not the former.

So then the aerobic athlete will always able to gain more quality training at threshold for less ANS stress with the optimal or ideal power Rx, where as the anaerobic athlete will always be incurring more stress (higher HR response/ANS response) if they desire to train at the same relative power OR face reducing the output for a more controlled HR and ANS response.

And then for specificity the former may be “shocked” when they need to go to the red in a demanding race while the latter has experienced it repeatedly.

I’ve seen this in these examples I coach and thus why I’m digging around a bit. Because I think it’s important to find a way for aerobic rider to experience that “red line” some in training as well as for the anaerobic rider to spend less time saturating the system.



All amazing comments for sure.

I feel like the athlete with the slow response, which I have more than the fast response, I have always used a series of races (2-4 weeks out from main competition) to sharpen this part of their fitness and mental capacity.

I do find some aerobic riders who can jump in and suffer no problem, and others can’t. This is where pre event planning may different between two similar athletes.

Great conversation! I’ll admit that I wasn’t fully following all of the questions. I didn’t know what you meant by "saturating the cardiac response. But, I hope this adds to the conversation…

The key thing that comes to mind for me is the fact that while heart rate is a very useful internal metric, it’s an indirect metric. It’s used as a surrogate for VO2 because it’s a lot easier to measure on the road and the heart rate curve correlates with the VO2 curve. But it doesn’t correlate perfectly. VO2 and lactate are our better direct measures - we just can’t use them on the road. What we have is heart rate - which is great - just remember that the response you see isn’t always perfect. Particularly when you’re trying to get a perfect measure of oxygen utilization.

I point that out because in the situation you’re talking about here - comparing a very aerobic athlete to an anaerobic athlete when riding at threshold, the heart rate data, while helpful, can be a little deceiving and requires careful interpretation.

What drives heart rate is the level of fiber recruitment and more importantly, the mitochondrial activity within those fibers. With the anaerobic athlete, when they start putting out a big steady power, they’re going to initially recruit more type II fibers with low mitochondrial density. Which means there’s going to be a lower initial driver of heart rate and they’re going to have a bigger oxygen deficit. Then even after heart rate has finally fully responded, because they are still more reliant on fatigable fast twitch fibers, they’re going to see greater fiber recruitment and importantly, much more fiber cycling. Hence the much bigger slow component.

By contrast, the highly aerobically trained athlete is going to see much more mitochondrial involvement right from the start, so they’ll have a much smaller oxygen deficit. They’ll also be much more likely to level off around their true anaerobic threshold. They are going to be able to rely less on fast twitch fibers and so the slow component will also be much smaller. You’ll see their heart rate and VO2 level off much more than with the anaerobic athlete.

This is always hard to explains, so forgive my chicken scratch, but here’s a quick visual:

All of this means that it is easy to over-estimate the first athlete’s anaerobic threshold power and heart rate. While, estimating the highly aerobic athlete’s true aerobic threshold heart rate and power from something like a 20 minute test is much easier.

So ,often when the anaerobic athlete thinks they are doing threshold work, they are actually training well above their true anaerobic threshold. That’s why the effort puts a bigger strain on their ANS. But if you were able to identify their true anaerobic threshold and tell them to train at it, they’d find it frustratingly easy and want to go harder.

When I’m working with one of these athletes who’s anaerobically strong but aerobically weak, I’ll often have them do a big anaerobic effort before doing the intervals. The idea is to get them to clear out some of the anaerobic energy stores and then have them do a truer aerobic effort. There’s plenty of research backing this and also showing that with these athletes, if you’re using a 20 minute test to estimate FTP, an all-out five minute effort before the 20 minute test is necessary.

When I’m working with a strong aerobically trained athlete, I’m often more tempted to just give them straight threshold intervals because they’ll see the very rapid heart rate/VO2 response and they’ll tend towards their true anaerobic threshold. I don’t find there’s as big a need with them to deplete some of their anaerobic stores first.

Hope that helps!


This is all super interesting! as someone who struggles to get there heart rate to rise to the occasion quickly enough on hard efforts and the awful feeling of needing more relief, I’ve often wondered what was going on to create this discrepancy… too big a heart? Low resting HR? Big lungs?..

Anaerobic athlete? I’d never categorize myself as one. Not a very great sprint, no super high wattage… But I can TT! So what gives? The differing opinions
above are getting me a bit confused.

Maybe this would make a great podcast to bring some consensus and clarification?

@trevor wow thanks for the detailed response and the drawings. Pictures worth a thousand words which helps clarify. I see this example that you elicit here but I’m referring to that as well as a different perspective. The slow HR response AND more so it not fully elevating.

To the point if I were coaching this person by HR only they would never be reaching Z4 HR unless we were doing MAP efforts and thus then they would either be too short or the absolute power would be “peak and fade”

Here is an actual TP image showing the a 3x15 effort hovering at FTP in Z4 and the 2nd image highlighting HR where each successive effort accomplishes more CD but still not ever out of Z3 HR. The accumulated TIZ based on power was ~50min of Z3 and Z4 combined versus 28min of Z3 by HR. And the specific comment was “HR seemed low for the leg effort.”

I’d usually like seeing higher power for given HR as I know personally this is when I know and feel like I’m coming around is when I can ride above Z2 power in short stints (being more of a mtber this is required by terrain) and still maintain a Zone 2 ride by HR.


@quadfather this is an interesting variation and why as a coach I feel each case and athlete is individualized. And I’d say this athlete is more in your shoes too and why I’m scratching my head. Not a huge power output guy, more of an aerobic diesel. And I’m using myself as a comparison to this athlete since I have quite a bit of metabolic testing and scans to back this up, I’d say I’m more anaerobic than he is, but my HR response to power is more like the “Aerobic” athlete in @trevor illustration.

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@dimatheny The graphs are very interesting. If they are doing 15 minute intervals and they are doing them at the right wattage, and most importantly, all else is equal then they should be hitting and even exceeding their threshold heart rate.

Without knowing the athlete, there’s two explanations that come to mind for me:

If the lower heart rate is just in this one workout and not every time, then there was likely another factor pushing their heart rate down. The most obvious one is that they were fatigued and might have been better off doing a rest day. The fact that they even commented that the heart rate seemed low for the effort indicates this might have been the case.

That said, I’ve had times where I’ve done threshold intervals, my heart rate was 5-6 BPM lower than normal, and I wasn’t fatigued. On those days, I finish out the workout and just accept the lower heart rate.

However, if this lower heart rate is happening every time, then they may very well be going too easy and need to raise their wattage to get to their true threshold. In other words, their FTP may actually be too low.

The problem is that you mentioned they are very anaerobically strong. If that’s the case, then getting them to do these intervals at a higher wattage may be tough. They could really struggle with it.

Since they are doing a lot of work (45 minutes total) and each interval is 15 minutes in length, I highly doubt that they are relying that much on anaerobic metabolism by the second and particularly the third interval.

Athletes with a lot of anaerobic power tend to prefer shorter efforts and really don’t like steady efforts. So, even though they might be capable of doing the intervals at a higher wattage, staying steady for that long is really outside of their comfort zone. Getting them to also work right at their aerobic limits, from my experience, is next to impossible with this type of athlete.

What I sometimes do with an athlete who relies a lot on anaerobic metabolism is give them over-unders. So each “set” is 15-16 minutes in length, but I have them alternate 2 minutes just above threshold and 3 minutes below. They’re just more comfortable when they have the variability.

Hope that helps!


![Captura de pantalla 2022-12-07 a las 8.01.50|690x263](upload://ooe4xdFTBYhgKMIN8FYg40vpIfv.jpeg
Greetings to all. Super interesting topic and leaves very useful reflections. I fully understand the concepts that @Trevor explains and they help me to understand the context of the cardiac response of a certain athlete profile. The attached images show an ftp test for two different athletes but with the same profile. In theory they are athletes with a powerful aerobic profile, however the execution of the test is totally different for both. One barely has an O2 deficit and keeps his pulse on threshold very well and the other is completely different. My reflection is that in the second case the athlete is able to maintain a stable power above his threshold for a long time and hence the behavior of his pulse is similar to that of a more anaerobic athlete.
My question is the following: Why do we see athletes capable of maintaining power outputs above their ftp for a long time? When you do FTP series, any consideration to take into account?
Thanks in advance and looking forward to seeing the answers

Thanks @trevor but I think you misread, this athlete I’d quantify as more aerobically based. His power profile doesn’t lean toward anaerobic. But this is a common occurrence for threshold and below;

Even endurance rides he can sustain power that is at the top or above Z2 for a Z1/Z2 HR.

But when it’s dynamic like a mtb race or a challenging road drop group ride that rolls from the OTC down here, his HR will get quite high to levels we don’t see in training very often. So all it takes is those dynamic punches to prime the CV.

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HI @dimatheny, sorry for the misread. Interesting about his response in races, though not surprising. I’m a highly aerobic athlete as well and I can hit heart rate and power numbers in races that I can’t touch in training. I’ve always felt that some of it is the punchiness but a big part of it is the adrenaline and mental motivation I feel racing. Some of us can just hurt a lot more in a competitive situation…

super interesting. This actually relates to a question I had after doing a 2x20m on the trainer before the weekend. Following some reduced load after the season + 3 weeks of sickness I have picked up training again. I did a 25 min race early last week where I averaged 236w and then I decided to do the 2x20 at 225. So, when I did this the effort felt way too easy for 75% of the intervals and with low heart rate but the last 8 mins I had to push a bit more. I know it is a “it depends” question, but how hard should the FTP work be, and was this too low? So I will try to increase the watts for the next time, but by how much… any thoughts?