Problem figuring out my Fitness Signature

I’ve got a problem getting a believable Fitness Signature and I’d like some advice on what’s wrong, if anything? Attached you’ll find three workouts I’ve made in the last couple of days, shown using the same Fitness Signature (determined by the 40 s interval session). The thing is that I’m strong on short intervals, but after a couple of minutes I’m much more fatigued.

The 8 min session should hardly raise my breath if I understand things correctly, and while I didn’t go for complete exertion I felt quite tired in my legs and didn’t have that much more to give.

The 40 s interval workout was hard, but I could probably have pushed even a bit more. I got a solid breakthrough and raised both HIE (21,4 to 24,6) and TP (313 to 325) on that session.

Then I did the 4 min interval session yesterday. There is no way that I could have pushed much harder than that, just check my pulse, but given my latest Fitness Signature this should have been a walk in the park. This was my best 4 min interval session this winter and even though I’m actually far from what I was able to do a yea r ago, I feel that this is rather representative of what I’m capable of doing right now. What I’m saying is that I don’t think the result should be due to tiredness, not entirely at least.

How can this be?
8 min 40 s 4 min

Your 4 minute effort are actually quite strong. Bringing MPA that deep is near your limit. MPA reduces non linearly so once you get to 800W, it will decline much faster. If you wish, you can look to validate your numbers if you don’t stop at the 4 minute mark. Keep going and you’ll see whether you can bring MPA right down as you did with the 40s efforts. 8 minute efforts at TP feel much harder if you have any tiredness in your legs. Similarly, if you’re 100% fresh, 5W below TP seems much easier. Since you’re doing intervals in repetition and not always to exhaustion, the software may not be getting precise information on your failure points. You may need to push longer at the various intensities, when you’re fresh, in order to establish a more accurate representation of your fitness. Great work, btw! Those are tough workouts!