If i haven't done a max sprint in a while, will Xert overestimate my FTP?

Or have I misunderstood the MPA calculation? Xert thinks my peak sprint power is 903w - though I know it’s higher than that and I just haven’t done any max efforts in a while. Will this cause Xert to overestimate threshold power as the MPA drops down closer to your apparent breakthrough effort more easily?

It can and it depends on the nature and specific data pattern of your breakthroughs. You should look to reset your PP first and then look to establish your TP. You can do both in a single activity if desired.

Would that look like: Maximal sprints, like 2-4 of them? then rest and do a longer steady state >TP until BT again?

Yes. Sort of like our fitness test workouts.

I find it impossible to get a BT with a steady effort of any considerable length. I can only do it while sprinting, and I’ve only done it in all out sprint at the end of a Zwift race (I haven’t raced in real life yet), or by performing repeated all out sprints to draw down MPA before bursting into another sprint. That’s what I did today, and my TP went up by 14W, which doesn’t feel reflective my actual ability, so I think depending on specific situation and calculation, it might. For reference, Xert thinks that my latest effort today (done with a few all out sprints) took me from 877/19.7/221 to 918/22.2/235, which seems rather generous, while GoldenCheetah, with identical history, thinks I’m 1001/14.2/218, and today’s effort didn’t change it. So, it’s really hard to know what is happening inside these black boxes and how to interpret what models are calculating.

Marko, it is difficult to compare since the approaches are very different. GoldenCheetah and every other software looks for best average power efforts (mean maximal power or MMP efforts) and unless they are paced perfectly from fresh to exhaustion, they won’t match what the Xert model obtains. Getting BT’s of extended length are harder, particularly when you haven’t properly warmed up and primed your system. Repeated efforts has the benefit of both priming and reaching exhaustion but only Xert can obtain your fitness from these efforts hence the results can’t be compared.

Golden Cheetah also has a W’ balance feature that in the event of a breakthrough effort will advise what your minimum FTP will be for the set W’. The graph looks like your MPA curve except when W’ Balance goes negative it is a breakthrough.