How is XSS calculated? Is it independent of fitness signature?

The XSS calculation uses your fitness signature in two ways:

  1. Your fitness signature is used to determine MPA second-by-second in your activities and strain is then calculated as a function of power and its proximity to MPA - the closer it is, the higher the strain. This is second-by-second as well. Strain is then cumulated for the entire activity. To be specific, each per second strain value is separated into low, high and peak strain and those are cumulated.
  2. Your total strain values for the activity are then normalized with your fitness signature to determine XSS. These are separated into low, high and peak XSS with each activity where they used in fitness prediction via three separate Impulse Response models.

To finish… The normalization of strain with your signature is to make the number independent of your fitness signature. 100 XSS means 1 hour at Threshold Power, unfatigued, for every athlete. Normalization also factors in your other parameters. The greater your Peak Power, the greater your capacity to generate XSS, for example.

XSS seems to work pretty well. I have now trained 3 moths against XSS and to me it gives much better approximation of workout “hardness” than other methods like strava, TSS and TRIMP. Especially the ability to count in short hard efforts seems to make it work. What I mean is that for example doing 100XSS workout is closer to being as fatiguing regardless of the time I spend to do it compared to other models. One thing where it seems to miss, like the xert system in general, is efforts near but below TP. For example riding 30 minutes 5W below TP is very tough even if I could on a good day do full hour at TP. So it seems to be possible to “cheat” or to get easy XSS by riding mostly below LTP and doing short efforts above TP. Getting the same XSS feels much more difficult by staying between LTP and TP all the time. The same effect is visible in the MPA chart when even a long ride just below TP looks like you made no effort at all (MPA does not drop). I would really appriciate if you created a better model for that area between LTP and TP. Here is an example of not so long e ffort mostly between LTP and TP where in the end I gave everything I had but didn’t get close to MPA. By lowering my TP by 10W (and getting higher HIE) it would look correct (MPA and watts meet in the end) but then that TP would be too low for other kinds of efforts like one hour constant power. Xert1

Thanks Armando. I’m looking forward to try the hardness tests. I did a racy workout last Friday and reached about 160 XSS in an hour. I guess that’s ground for me to try hardness test level 11 or 12?

And then all the exciting Smart workout too. Tried one but more to come!

I agree with Arto - the time spent in the zone between LTP and TP is tougher than it appears in Xert. I would like to be able to separate strain from LTP and lower and strain from LTP-TP. This would be probably be useful when calculating the amount of training I spend according to the 80/20 rule.

But overall of course, Xert is really great. Love it

There are a few factors at play when it comes to the zone between TP and LTP. Firstly, some have called it the “black hole” and in general should be avoided in training programs. Of course, there are opinions and caveats to that. But this does suggest from a cumulative strain perspective, if this in fact aligns with fitness, that one would expect strain to be harder to accumulate. Secondly, this zone is where you move from fat to carb energy sources. Spending a lot of time above LTP is going to reduce carb stores more rapidly per joule of work and this will lower your TP in proportion to work performed (so yes it will get harder and one would see you TP decline, even in as little as 20 minutes near TP). Thirdly, central fatigue also may come into play more in this zone making it feel harder on a relative basis. XSS isn’t accounting for central fatigue, at least not directly. This is why Hardness Tests may expose a central fatigue weakness even when MPA is near PP.