Question about how Xert calculates Fitness Signature

Did a group ride today, some hard efforts but nothing I haven’t done before. Xert when nuts when analyzing the ride, Yesterday my Fitness Signature was realistic and in line with both actual test efforts (for 5 minute power) and traditional estimating procedures for TP. Today’s ride boosted TP 6% and 5 Minute power 5% to completely unrealistic levels. This again raises a nagging suspicion I have about how Xert does its calculations, please correct me if I’m wrong: certain pieces of power data Xert extracts from a ride are compared some standard model to imply all other power durations and peak power, and that the model(s) you’re using to compare actual data snippets to don’t take into account other important individual factors like age.

Scott mentioned, in reply to another question a couple weeks ago, that Xert’s model for MPA decay and recovery applies to very high level/professional athletes but you realize that for regular amateur athletes MPA doesn’t return to it’s maximum value as fast or as completely as currently modeled, and it’s something you’ll be addressing. Which implies to me your modelling probably ignores age considerations as well.

Whether I’m close or far afield about the supposition in the first paragraph, for Xert to be really useful to me it would have to tailor how it comes up with all the parameters in the fitness signature with regard to the effects of age (and get the in-ride MPA decay/recovery correct as well since your “brakethrough!” analysis is based on it). You can’t imply an Xert Fitness Signature from bits of extracted data to 50, 60 and certainly 70 year old riders the way you do to 30 year olds. If in fact you’re trying to tailor it now you don’t quite have it right.

Hi Ken. Pretty much guarantee that your Peak Power is underestimated, especially given your last 2 breakthroughs were at such a high power. Everything before then was spot on. See the FAQ on checking your signature. You may need to perform a true peak power effort (suspect it’s likely as much as 200W too low for you). The Xert method has applied to many thousands of athletes at every level that use it regularly. Sometimes on conditions such as yours do errors creep in, but they are easily corrected. If you need further help, contact our support team via and they’ll sort things out for you.

I’ve never seen an instantaneous power, 1 sec.-no smoothing, at the level Xert currently has for me let alone 200 watts higher.

From the links you posted maybe my HIE is too low in regard to how Xert calculates TP, but wouldn’t a higher HIE raise my 5 minute power estimate to an even more ridiculous value? Anyway, how do I manually modify the HIE estimate?

Can you answer my questions about whether Xert uses a one-size-fits-all model when calculating a fitness signature and power duration curve from power data bits and pieces?


Hi Ken. Thanks for your questions. Every signature is unique to the individual. It’s not a one size fits all sort of thing. We have every age using the system. Generally speaking, older adults will have lower signature values because they don’t have the same power. If your HIE is too low, then your TP would be even higher with a higher 5 minute power so that’s not likely to be it. Q: What do you expect your 5 minute power to be? Your recent breakthrough had a 6 minute power roughly 30W above your TP … and that was even a highly variable effort starting with existing fatigue. If you think that this doesn’t make sense for you then, you should check your power meter since this is what is it giving the system. The system doesn’t look at anything other than your data and accounts for fatigue in assessing what your signature is (and thus what you could sustain for 5 minutes). Perhaps this new 5 minute power is sustainable with the power meter you’re using in its current state? Btw, you did have a nice steady increase in training recently so seeing higher numbers than you have seen in the past is likely expected. Good work.

Send a note into support so we can communicate with real numbers and explain a couple of things. Cheers.

I expect my 5 minute power to be about 318, which is in fact the value Xert had for me yesterday (nice work yesterday Xert!). There’s two things here re HIE, what it is and what Xert thinks it is. If Xert thinks my HIE is lower than it really is then that will raise the Xert calculated TP higher than actual; and today the Xert TP is too high (yesterday it was within reason). So if my Xert HIE rises (toward what it actually is) it will lower my Xert TP, correct? And won’t a higher Xert HIE raise my 5 minute power even more, just asking?

OK, I’ll send a note in to support. BTW, I have three power meters, Assioma pedals, Quarq Dzero, PTap G3 hub and they’re all trued up and agree with each other, taking in to account the mechanical losses the G3 sees the other two don’t. I do a zero offset before each ride. So the numbers are good.

Don’t forget that fitness does change week to week, something that most other systems don’t pick up. You’ll see this in Xert once you get into it more and see when breakthroughs come. It helps to keep power data consistent. Even though you’re calibrating and have great PMs, there can be discrepancies. Thanks for your great questions! Keep up the great work.