Heart Rate during threshold intervals

Hi All,
Is that time of the year to start doing 3 x 10m in the trainer at threshold. I think I can start the progression 10w below peak FTP last year. I’m used to doing longer threshold intervals up climbs (30-40 minutes), so not very familiar with 3 x 10m indoor. My max HR is 195 and Threshold HR is 175.

When I do a long threshold effort, it takes me about 30 minutes to reach a HR of 175. So my question is: When doing 3 x 10m at threshold, what is the expected HR response relative to HR threshold?

** I know HR is individual, that’s why I’m asking the normalized response, as a fraction of HR Threshold.*
** I know HR might vary from day to day, but in general it’s pretty consistent when controlling by stress, heat, sleep, hydration.*

I went for a 4hr ride with some triathletes last week, where my HR was above LT for 1/3 of the time. It was ridiculous, I can’t reconcile it other than a) threshold is my kryptonite b) I was sick, tired or both c) wtf ?

I think it is what it is. Ride a 3x10 at threshold and see your HR response.

For me, if I was to do something like this, my avg HR for each interval would increase noticeably vs the prior interval. But not reach the same HR for as long compared to a 30 min interval (at same power).


I did 2x20 the other day and never quite made it to my LTHR… though Ive also been wondering if my FTP is 10-15w higher than what I’ve been using. So they may have been more like 95-97% of FTP.

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With all the standard caveats about day-to-day variability, hydration, etc, I think that taking 30 minutes to reach LTHR/ not reaching it in 3 x10 min efforts is a sign of being well-conditioned to those type of efforts. I’d be asking what your RPE was, and what was the pw:hr for those efforts; that might tell you more.


I think there’s an obvious question here if: are you sure that’s your FTHR or, are you sure you were actually pushing threshold power for those periods?


My rule of thumb is that threshold heart rate takes a while to reach, lets say doing 3x10 towards the end of the second interval and then it reaches a stable plateau

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Are you happy with the answers @oldandfast ?

Below is a 15 minute grab from my 4hr ride where I’m riding under threshold power, but at 103% LTHR. Does not compute

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@oldandfast if it is taking you 30 minutes to reach your “LTHR” you are not riding at your threshold… You are likely riding sub threshold watts and the duration of the effort is causing cardiac drift upwards to your assumed LTHR.

Have you have done some type of FTP test recently? use the power results and disregard HR preconceptions. 90% of max HR isn’t reliable unless your highly conditioned and you can actually hold that 90% of max HR with stable power for longer efforts approaching an hour.


For me it would take less than 10 mins of riding at threshold power for me to reach my threshold HR I think?


My Max is 197bpm and whilst I go on that intervals calculates my THHR at 181bpm and looking at my hour + TTs it seems to be appropriate.

On a club TT with 15mins warm up on the road and 5 min pause on the start line I’m hitting that at around 6.5-7.5mins.

On my last 1h + TT with a 20mins rollers warm up and probably 10-15mins ride to the start I was hitting that after 12.5mins.

Indoors though I am seldomly reaching that on a steady state wo (typically around 175bpm max) from a quick scan the closest I have got to threshold HR recently was on a 3x15min SS work out where after a 25mins warm up I hit 178bpm at the end of the first SS interval and 180bpm at the end of the latter two 15min SS intervals.


I concur with @HLaB’s findings! IMO, you should also look at this being indoor vs outdoor.

Personally speaking, for the same threshold wattage effort, my HR (Max and LTHR) will always be higher OUTSIDE, due to all the external factors that come with riding outside (wind, rolling resistance, rigid trainer vs fluid bike movement, etc). While I can hit my 175 Max outside during a threshold effort, I VERY RARELY eclipse 160 inside… and that’s only for a second or two at most and i’m right back in the 140-150 range again.


Thanks. I’ve been thinking about this. Since this is indoor and not my favorite, different source of power…etc. So it’ll be power and RPE.

Good questions. LTHR number came from a good test, but it’s possible that it has drifted down. I haven’t done a good test in a while, and trying to avoid doing one indoor…so that’s why I’m trying to calibrate with HR, it might be futile.

Yes, giving me good food for thought. I’ll be doing the intervals today and see what’s up.

Your response resonate with me, and gets to the gist of my question. It seems that you have some estimation of how long does it take to reach LTHR while riding threshold, can you please share that reference? Thanks

Thanks @HLaB for the data points.


  1. There are many sources of potential error…my estimation of threshold, the decay in the calculation of HR Threshold, indoor vs outdoor.
  2. I should do the work and pay close attention to the descriptive metrics and assess.
  3. The dispersion in well calibrated data shouldn’t be that big between athletes.

Will report findings, thanks all.

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I honestly feel like the best measure of FTHR is perceived, a little like the talk test for Z2.

It’s the ‘im fucked if I stay over this number’ test.

I know mine is 171. If I’m going for a KOM (just as an example of a capacitive effort), I know when I start creeping into 173-174 I’m running on limited time, and when it hits 175 I’m going into the red. Over that and I need to actually rest to bring it down. But if I stay +/-1 of 171, I can keep that going for a period that I consider functionally ‘long’ - 30-40mins.


What test did you use? The standard is like avg 20min hr of the last 20 mins of a 30min threshold effort, so kinda by definition you should reach your LTHR during the first part and it should slowly drift up

Last 20min of a 30min TT, so probably a bit above FTP.

This requires reasonable pacing for a 30min TT effort though. If you go too hard early then back off, it theoretically underestimates LTHR.

Garmin and intervals.icu both are quite accurate in their estimation of LTHR, so are reasonable to use as well.

Intervals derives it from 20min efforts or longer I believe, whereas Garmin bases it off of measuring your VT based off of changes in HRV. This means you may need slightly less data for Garmin to give you a good estimate.

Average HR for a 1-h effort is also another method - so do Alpe de Zwift as hard as you can and take that.

There is also good correlation with just paying attention to your breathing. When it transitions to out of control panting, you’re above LTHR. Easy to notice what HR this breathing changes at when you’re doing something like a set of 3-5min VO2 intervals.


Yeah. According to these, mine is more like 173