Just done a zwift 60min meetup with a friend. We both did the same workout (Smart Headstrong 60 based on XSSR) with trainer control from the IOS app. Our sigs are different 775-15-205 and 1080-23.5-245 but we somehow expected the workout to deliver the same relative level of strain / effort. However we discovered the workout XSS value was higher for the stronger sig and duration longer - which kind of puzzled us?
You can test this out with the Workout designer by entering a different fitness signature and clicking ‘refresh’.
For example, an athlete with a signature of 1000 W PP | 25 kJ HIE | 250 W TP:
And an athlete with a signature of 1000 W PP | 15 kj HIE | 250 W TP:
It also slightly depends on how closely the trainer controlled your intervals, since the ‘recovery’ intervals are defined as 75 XSSR until 99% MPA Reserve is reached. Since the athlete with a larger HIE will take longer to ‘refill’ their MPA, they will perform more work… more work = more strain.
This is a great example of how Xert tailors workouts uniquely for every athlete (and adjusts in real-time!) … something that can’t be done anywhere else! 
Hope this makes sense!
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Many thanks for the helpful reply Scott. I did play around changing the values in the workout designer and noticed that HIE influenced the XSS value and duration. So on a perception level should we burn both be equally tired
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This is a practical demonstration of Xert’s SMART workouts.
They’re going to vary in time between athletes (and even adapt in real-time, depending on the execution) but ensures that every athlete reaches a specific point of fatigue (or recovery), according to their unique fitness signature, before proceeding to the next workout step.