Hi Jeff,
Excellent questions, and the discussion has been really good so far. I also just want to chime in with some thoughts & comments from our end at Xert.
By definition, threshold is the highest power that you can sustain that results in no net change in MPA (i.e. MPA will neither drop nor recover while riding at TP). As @Cyclopaat has already mentioned, for many athletes, the TTE at Threshold Power is typically around 40-70 min.
DS is an EWMA of XSS/hr with a time constant of 30 min. Since riding at threshold (with MPA = PP) is 100 XSS/hr, the difficulty will likewise top out at 100. If you try changing the workout to 4 hours at 101% TP, you’ll notice that this will change things a LOT.
As of right now, all XSS below TP is considered Low Strain (you can see that in this image, taken from our blog post on Work Allocation Ratios). Notice that below TP (250W for this athlete), it is 100% low strain:
However, its entirely possible that we haven’t fully modeled this correctly, and that there may actually be non-linear allocation of high (and even peak) strain when riding below TP, which would align with why some athletes find Sweet Spot training so effective.
It IS entirely possible to hit MPA & get a breakthrough 4, 5 hours into a ride (I’ve seen plenty of times), but it does require very conservative use of hard efforts earlier in the ride, as well as adequate hydration and nutrition throughout the ride.
Hope this provides some additional insight. Cheers!