Power/HR in Z2 metric

Intervals.icu now calculates power / HR for time spent in heart rate Z2 for each ride. This is the average watts / average heart rate for the parts of the ride done in HR zone 2 (aerobic or endurance). The ride is split up into 1 minute sections adjusted for HR lag. Only minutes in HR Z2 with cadence between 85 and 100 and little decoupling* are kept. The power/HR Z2 ratio is the average of these minutes and an indicator of aerobic fitness.

You can add this as a plot to a custom chart. If I look back on 2 years of my data the times when power / HR Z2 has been highest are also when I have had my highest eFTP.

  • I still need to implement the “little decoupling” filter.

I usually drive in Z2 at 70-80 rpm :slight_smile: It doesn’t work for me :slight_smile:
I need to change my pedaling technique


60 - 70 rpm here - I have a 55 chain ring and only use the 11-14 on the cassette :joy:

Oh wow. Didn’t expect that. The cadence filter is supposed to filter out coasting, cadence drills and so on. Not monster mashers.

I’m in the same boat: 85 rpm is about where I sit in Z2.
Perhaps use a power filter instead? I mean, define Z2 by HR as you have done, but then filter by [say] watts > 50%FTP rather than cadence? That gets rid of coasting.
It is almost true that whatever cadence range you select as a filter, there will be people who regularly ride outside it. Perhaps just drop this and advise people to bear it in mind when interpreting the results?


Same here, I’m usually around 75 in a chill Z2 ride, 85 seems to be my sweetspot for sweetspot stuff, 90 for threshold, 90+ for above threshold and 95+ for racing, where I tend to spin more at all intensities for some reason, adrenaline maybe. I’ve also noticed it changes a bit over time. I’d like to think I’ve gained some more muscular endurance this year which enables me to be more comfortable in lower cadences/more torque than previously, while I’m still able to spin when needed, but that’s just what I like to tell myself, maybe it’s bollocks :slight_smile:

I am working on this. My data is very messy without the cadence filter. I will try a cadence window related to the mean for the Z2 parts of the ride to chuck out the outliers.

Maybe insert cadence filter configuration?

I could but its better if I can come up with something that “just works”. I think filtering based on cadence of the ride in Z2 HR will do the trick. If you normally ride at 80 rpm it will use say 75-90 as the range etc.

Clever pants! So what’s your thinking here – +/-10 per cent? Is that wide enough? I suppose that we are talking about Z2, so that might be enough.

Wassup with Z2 anyway? I only spend 26% of my rides in Z1-Z2, so even if that’s more than 50% Z2, it’s still (way) less than 25% of the total…

I guess my zone discipline pays off, this was a ride from last sunday. I just upped my FTP so I’m still getting the feeling of my new power zones.

Is usually end up with something like this…

It now finds the median cadence of the Z2 HR data points and keeps those within 0.92 and 1.15 of that. I will deploy that tomorrow morning to recalculate everyones stuff. Its a reasonable match for eFTP for me for the last 2 years and nice because you don’t even have to do max efforts for this, just ride easy Z2 and you can figure out how fit you are!


I also added CSV download of the HR lag adjusted power vs HR minutes for an activity:


Out of curiosity, would it change the graph at all if you switched the zone filter to power? Thus plotting Pw/HR for time spent in power Z2.

Probably not much unless your power zones have changed i.e. FTP changed. The big advantage of using HR zones as a base is that they generally don’t change during a season.

They do as you fatigue (too much) in which case HR is either lower or higher than normal.

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This is very cool. One thing I thought of and maybe it’s already factored in or doesn’t matter, but sometimes the entire ride is Zone 2, and sometimes the majority of the ride is 1 hour intervals with .5-1 hour zone 2. I find that Zone 2 heart rate after an interval session to be higher than just plain zone 2. Does this matter? Would looking at only complete zone 2 rides show a better indication of fitness increase?

No. You will fatigue more during the intervals, after which it is not uncommon to end up with a slightly higher than normal HR, compared to Z2 only activities.