Outside test vs Inside Training

I went out last weekend and did a 60 minute test. Heart Rate was approx 90% of Hrt Peak for the 60 minutes.

When I use the power from this test as my FTP on my indoor trainer for my 4x8 intervals I am nowhere near my 90% Hrt peak threshold - its much lower, close to 80% of peak in fact. My indoor trainer is calibrated.

My RPE at this 80% power on the trainer feels about right maybe a touch low but not hugely far out. Heart rate drops to sub 70% prior to the start of each 8 minute interval so i’m easily handling the trainer power.

Do I increase power for my 4x8s until my hrt in the last couple of intervals is closer to threshold hrt/90% of peak ? @ryan ?


Are you using ERG or non ERG mode inside? Secondly, is there a cadence diff between your indoor and outdoor rides? Sometimes a lower cadence will keep HR lower, this is why I ask as HR is response to what are feet are doing as well as the effort/power level. All things equal be sure your FTP is updated on indoor app like Zwift etc is using ERG mode.

Are these 4x8 and HR metrics you speak of related to polarized training? If so, just focus on the HR and get to 90% of maxHR in the intervals and then hold on and sustain the highest power or effort level you can in order to keep HR at 90% and preferably rising a bit to 93-95%.

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Thanks Frank for the advice much appreciated.
The difference is so big I have a feeling it might have been the way I did my outdoor test - its been my first for a long time so for pacing I targetted 90% of known heart rate peak for the hour and emptied the tank in the last 20 minutes. In addition the temperature was less than zero degrees Celsius and I did get a warning on my Garmin head unit about the low temp - unsure exactly what it said - font size was too small for me to read! But something about data not being accurate ? I use Garmin XC200 Rally power pedals not sure if the warning was for the pedals or the head unit. Instead of finding an excuse to stop the test I powered on and thought lets do this no matter what, even if I could compare my distance with last time. My ear lobes were stinging with the cold !

Yes the HR metrics are related to achieving the Seiler 4x8s that are highlighted in the Polarized Pathway. I will crank the power up to achieve the target Heart Rate range you have suggested. Make sense. I’ll do another 60 minute test in a few weeks and also a power test on the indoor trainer an compare results.

Again thank you for your time taken to reply



If you read carefully the famous Seiler’s paper you will notice that 4x8 intervals are executed on average at 106% of a 40m TT power, so you should definitively not use your 60m power to do them and expect to go above 90% MHR. Keep in mind that even when done at 106% of 40m power the average HR for the 4 bouts will be (on average) 88% MHR and average HR peak 91%.


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My understanding of that 4x8 research of Seilers, published in 2013 in the Scandinavian Journal of Medical Science Sports, is that the effort for each interval was isoeffort (the italics are his in the research paper), which was maximum maintainable effort.

One thing puzzles me, perhaps you or someone else could clarify: If you do sustainable maximal effort for 8 minutes, then your HR at the beginning will be significantly less than in the last minute.

Seiler refers to approx. 90% HR for 32 minutes. 8 minutes 4x. You would get perhaps an average of 90%, but the nature of the effort suggests that you would have a number of minutes under 90%, rising to quite above 90%, to reach an average of 90% HR for the 32 minutes.

Let me know if I am missing something here. I remember years ago Trevor and Chris talking about if you are at 90% HR early in the repetition, especially in the first one, you are going too hard

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You are correct! The prescription in Seiler’s paper is not wattage but the effort that will allow you to complete the 4x8. The power % I am referring to, is the result of that prescription, so it can be used as an estimate (or first approximation) on the Watts you should/could sustain for the workout. It is of course an average and the standard deviation is high (+/- 8% !). You are also correct about the HR. Average and peak HR for every bout will increase. Everyone will have different numbers; in my case I can execute the 4x8 at 103% the power I can probably hold for 60m, average HR for bout are ~ 84, 87, 89, 91, total average ~87.5 (so pretty much the 88 documented in the paper). Peak HR for bout are 88, 92, 94, 95, average 92 (again roughly the average 91% documented in the paper). Cheers!

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Thanks for the excellent clarification!

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Really appreciate the replies to this - thank you team !