Stephen Sieler on cardiac drift, power/HR and so on

So Stephen just released a new video with some experiments he has done regarding cardiac drift, especially during long zone 2 sessions. He compares and uses different ways to relate HR to power (and not just power over HR), talks about fatigue and some other interesting stuff.

Though I could share, as @david has been focusing lately on this subject.


Tx. Wow interesting video! Might be a IN/EX_S ratio chart coming at some point :slight_smile:


If I implement some of his ideas and he notices I might have to ask Strava for higher rate limits :slight_smile:


And us for more contributions :sunglasses:

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It’s a deep rabbit hole that I’ve been going down lately. I’m looking forward to watching the new video.


Daniel_Hernandez: can you provide the link? I cannot find the video. Thanks.

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Thank you, David, for the video.

I know the feeling I do similar things. I’ve just plotted on google sheets my last endurance ride split into 20min intervals.Endurance April 22nd 2020 159W_130bpm


@Cary_Blackburn If you have some takeaways from your experience with crunching those numbers I would encourage you to add to the thread I linked. It’s always helpful to pull from as many experiences as possible. New perspectives on what data is useful and how to display it will surely help with developing features.

Thanks I’ll have a look at it. Perhaps @david could possibly reach out to Stephen Seiler about the possibility and/or best way to calculate/ present this concept?

I think I can probably get something going from the video, then ping him maybe. I am sure he is pretty busy and gets a lot of pings!

He uses % of 6min power as the power measure, but I think % of FTP would work just as well and better fits the FTP-centric nature of The HR measure is % of HR reserve. I could add a chart with those on it to the ride timeline. So much to do but think this one is bubbling to the top of the heap right now.


I don’t think he’ll be that busy - he’s stuck at home like the rest of us!. He might even welcome it.

FTP as target is debatable, as that is different in time / duration for many, while 6 minutes is, well… 6 minutes.

I suspect from his point of view FTP is too inaccurate across a range of people whereas in his academic world 6 min power is a close proxy for Maximum Aerobic Power. On an individual level FTP would be fine I would think. It’s just really something to anchor and compare the HRR/HR% to.


You are just much more articulate than me :joy:

Actually on the activity power page would be better, thats where all the current decoupling stuff is. And then it can be a bigger chart with some explainer text and link to the video.

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I agree, and also agree that it can be done with the FTP that ICU calculates from our efforts instead of the best 6m power, as very few people actually do this kind of test or aim for a 6m best power.

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As this is about endurance LT1 Power & HR and would be potentially of most use during a Base phase when you are perhaps not doing any maximal efforts then the ICU calculated FTP would probably be too low and would drift downwards. The FTP that you can alter could be a better choice.

I definitely agree with what (I think) @Bogdan_Rylski is saying in the other thread about 20min Z2 intervals. Without very carefully set parameters, this could easily become a “garbage in / garbage out” metric, where 99% of athletes are inputting garbage.

I pay attention to low intensity training and do longer rides at <70%maxHr as Seiler and others suggests. But I rarely do more than 90 minutes on the turbo where you can get the consistent output necessary for reliable analysis. And even the 90 minute sessions I do (Trainer Road) have some fluctuations in power, rather than just riding at a set HR or power.