Is TSS of Sub-threshold Zones/Levels Created Equal?

Usually when ppl invoke the expression “not all TSS is created equal” they are referring to different sports (“running TSS isn’t cycling TSS”), or within cycling how TSS from supra-threshold (VO2max, anaerobic, etc.) cannot necessarily be equated with the same TSS derived from several hours of .60 IF riding. Different adaptations, different recovery, etc.

But if you limited it to Zone 2 - Zone 4 (maybe Zone 1 - Zone 4), is TSS created equal? Is it then an apples to apples comparison?

Just to kickstart some discussion (and hopefully further my understanding), if this isn’t the case then what is the point of the metric?


I always go back to think about the relative difficulties of riding an hour record at an intensity factor of 1 and a 2 hour ride at an IF of 0.71 (upper Z2). Both have a TSS of 100, but 1 is much easier. They are not equal!



I understand that. When IS it equal?

Well, that really is the million dollar question and I’d like to do some analysis on that at some stage.

Regarding the true physical toll of lower intensity riding:

Seeing as Z2 is something that you should be able to sustain for many, many hours without too much need for recovery before doing it again, I don’t really get why you get 50 TSS per hour. More realistically it’s probably 25 TSS per hour or lower until you go over a certain time threshold - after which piling it on can start to cause real damage that takes much longer to recover from.

On a slightly different but related subject: TSS relative to other riders will also be wildly different depending on strengths and weaknesses. If you have two riders with an FTP of 300 Watts and the both do the same VO2max workout at 360 watts (120% FTP) they will both get the same TSS. However, one rider may be able to do the workout at 130% and the toll on their body will be significantly less than for the rider that did it at their maximum. TSS should be lower for the stronger rider.

I’ve been thinking that actual power needs to be ‘normalized’ (not to be confused with Normalized) so that the rider with VO2max at 130% FTP gets that effort scaled down to match the ‘standard’ 120% (or whatever it may be) before Normalized Power is calculated. The same thing would happen with the higher zones too, based on ability relative to the norm.

Obviously more maximal data would be required to do this, but that shouldn’t be a problem so long as it is truly maximal either from formal testing (something like Full Frontal) or from data collected from outdoor rides.

Certainly quite interesting when you look into just how generic the system is.


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TSS is just a number to show the accumulated training load. It makes no comment about how it was achieved. You can’t use TSS alone to plan training, but you can use it to avoid overload to your body.

If someone were to look at a weeks training and decide if they’d done a quality week just by looking at the TSS then they are missing the point. More important that the simple TSS score is what you have done to arrive at that number.


The point is that how it was achieved does have a bearing on how accurate the number is. Doing 400 TSS in Z2 wouldn’t be something to worry about but 4 hour record attempts in a week would destroy you. The load is not equal.


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I think that’s what I said in “more important than the simple TSS score is what you have done to arrive at that number”?

Look at what the score is, but ultimately your body is going to tell you more about how stressed it is than a number on a screen.

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That’s fair enough - I probably read your post too quickly as I headed for my train.

The question here is how could TSS better reflect the actual stress on the body, because, as it is, it isn’t a great measure when looking at different intensities.


Bingo. Very magical my man. You read my mind :smile:

And I would add it isn’t great when looking at different intensities even though that seems to be precisely what it was designed to do. Now, maybe we have a better understanding now (similar to our understanding and usage of FTP compared to when it was first introduced), but that’s why I’m asking the question.

One of the first steps could be to give all power in Zone 1 zero influence in Normalised Power. By definition this is recovery so can’t be stressful on the body. That would reduce Zone 2 workout stress considerably but have minimal effect on zones close to threshold.

I might do some calcs at the weekend to run through some scenarios.


It’s best to understand what TSS is before we ascribe our own characteristics to it, or manipulate it for ends it is not meant to achieve:

(Googled a coggan article on tss, I’m sure there are others)

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Maybe this is more appropriate?


Thanks @apond58. I’ll give the Xert articles another shot, but I’m very skeptical of the validity of their modeling. There may be some physiology to learn though.

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I was on Xert for a number of months and found it a bit too deep for me. Many of the data geeks on this forum could likely figure it out and put it into english for us self-taught Cat 5 physiologists.:face_with_monocle:

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An interesting comparison:

Xret stress score for the following 1 hour workouts:

100% FTP - 100 XSS
70% FTP - 66 XSS
50% FTP - 45 XSS
40% FTP - 35 XSS

TSS for the same 1 hour workouts:

100% FTP - 100 TSS
70% FTP - 49 TSS
50% FTP - 25 TSS
40% FTP - 16 TSS

Xert appears to make things even worse…


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you would laugh at my Strava relative effort values for my zone 2 rides.


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TSS, like most power training metrics, was developed by Dr. Andrew Coggan. As @AndyGajda mentioned earlier, TSS is simply a measure of the stress on your body from any given workout. It’s best use, as Dr. Coggan has shown, is part of the Performance Management Chart (PMC). The PMC tracks your individual workout (TSS and IF), your short term training load (aka ATL or Acute Training Load), and your long term training load (aka CTL or Chronic Training Load). [CTL is a good proxy for your fitness level]. Your ATL minus your CTL is your TSB, or Training Stress Balance (i.e. how fatigued you are). The PMC, as available in Training Peaks’ Premium version or TP’s WKO4, is a very powerful tool for tracking your fitness level and helping you determine whether you are getting enough overall stress on your body to grow or conversely have too much stress and need some rest. I’ve been using it for 3 years now and would not do without it.

Regarding not all TSS being equal, I think we all know that. But if you are looking to focus on adaptation, then you need to look at how much time you are spending in each of the training zones to meet the requirements of your A and B events. It’s called TiZ (Time in Zone). Similar to the PMC chart, tracking of TiZ is provided in TP’s WK04 allowing you to monitor time spent in, for example, Endurance, Sweet Spot, and Threshold as well as zones above FTP known as Coggan iLevels.


This is one of the things that motivated me to ask the question. Tracking TiZ seems like it might be an answer (the answer?). But if that’s the case, then TSS doesn’t really give me anything. I mean, I HAVE to be missing something very basic here, right? hahahaha

@bobmac I’ve been using PMC, etc. how you describe for about 2 years. I’m considering getting WKO4 (and have learned much watching Cusick’s videos). I’m almost ready to cancel TrainingPeaks Premium in favor of just using TrainerRoad bar chart thingie.

I think we do. And yet we keep using it. Why?

Just my opinion, but I think TSS is more valuable (and the PMC, too) for someone training higher volume. If you’re focused on driving up CTL and doing a lot of SS and Z2 it can be very helpful in quantifying just how much you can push. I know how far I can push my TSB into the negatives and for how long because I’ve done it and learned the lesson. I know Joe Friel says during a block to keep it between -10 to -20, anything less than -10 and +10 is doing very little and most people get cooked quickly past -30 TSB. Seems accurate.

If you’re on a lower volume plan with SS, VO2 and anaerobic intervals and not pushing much past 400-500 TSS per week I’d imagine TSS and PMC is less of a factor. Until you get CTL fairly high (80+) I haven’t found the effects of peaking to be nearly as dramatic. Take an easy week, do some openers and go race. Likewise, Z2 rides become regular aerobic maintenance when you’re at those higher CTL numbers.

While it’s not a sexy approach and may not give magical FTP gains, if you can stay healthy while driving your CTL higher, you’re going to be stronger.