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.