Xert Just Added ‘Durability’ a Pro-Level Power Metric

BaronBiosys, the company behind Xert, has introduced a new cycling performance metric – Durability, which gives a deeper understanding of performance degradation on long rides.

You are familiar with FTP/CP, but Xert works slightly differently with power data using W’ to create a performance signature of your abilities over different durations; a breakthrough ride would be where you exceed a signature value, which might have zero effect on your FTP. However, there is a further complication. Over long rides, you may not be durable, meaning that your FTP (or entire signature) will decline during the ride.

This metric links in (ish) with several articles I’ve written here, trying to convince you that FTP isn’t your hour power! People are finally listening – see Dylan Johnson, YouTube, for his usual entertaining and informative takes.

If you think about it, durability is fundamental to your long-course performance, whether a 100-mile Sportive or an Ironman.

What’s the point, or scientific basis, for targeting a percentage of your FTP for a 100-mile ride? It might be a decent approximation but…

Xert Adds Durability Metric

More specifically, Xert’s Durability tracks and evaluates how an athlete’s Maximal Power Available (MPA) decays over prolonged periods.

A higher durability score is better, as you have greater fatigue resistance.

Understanding Xert’s Durability Score: A New Measure of Fatigue Resistance

During extended exercise, your ability to resist performance decline encompasses multiple physiological processes, including muscular, metabolic, and neuromuscular fatigue. It is particularly relevant in understanding how power at lactate threshold (LT2) or VO₂max degrades after several hours of exertion.

Recent literature has emphasised durability as a better predictor of extended performance in endurance athletes than maximal aerobic or anaerobic values alone.

• 2021 review by Maunder, Seiler et al. in Sports Medicine –  performance modelling often underappreciates fatigue resistance. Seiler is worth following on X.
• Van Erp et al. (Med Sci Sports Exerc, 2021) – wide variability in fatigue profiles among World Tour cyclists during Grand Tour stages.

Durability in Power Analysis: Xert’s Latest Innovation for Endurance Athletes

Xert is not the first platform to attempt fatigue modelling. Other platforms include WKO5 and INSCYD – the latter has a Fatigue Index based on individual metabolic profiles.

When Xert was first developed, durability was part of the architecture.  Looking at the original post on Fitness Signatures, published in December of 2014, we described the concept of “Slow Recovery Fatigue”.  This is ostensibly durability.  It describes how your power duration curve changes over the course of a very long event.  We used this phenomenon to help define our concept of Lower Threshold Power.  which became a core part of Xert.  This was long before Zone 2 training became a fad … and now it’s Durability which has become quite prominent within training and coaching circles, especially at the elite and professional levels. [Xert]

Durability in Context

Durability is particularly relevant for athletes in events lasting more than two hours.

The Durability Score helps identify where and when performance decay begins, offering a clearer picture of an athlete’s “real” threshold power late in sessions or races.

One of Durability’s key benefits is its utility in structuring training. Coaches can use it to:

• Pinpoint the time or training load at which performance starts to decline
• Adjust training programs to delay the onset of fatigue
• Monitor whether interventions work
• Track adaptation without lab-based assessments

My Example Using Xert

Last year, I rode a relatively leisurely 200-miler in the UK’s ‘chase the sun‘ event. I’d never previously ridden more than 120 miles (Ironman distance). I found 200 miles no harder than some faster 100-milers I’d previously done, probably because I was going slowly and just aiming to finish. Nevertheless, I was tired!

I checked my Xert stats for the day in question.

Good. There were no Breakthrough performances on the ride. That means the durability calculation can work, so I clicked the ‘calculate’ button and then saved the result in Xert – not sure why! It will probably use it in another calculation in the future! But I was horrified to see that Xert had my threshold power declining by exactly 25% by the end of the ride. I guess that’s plausible.

Note: I’ve used Xert for years. Its threshold modelling feels the most accurate of all the tools I use. Golden Cheetah gives similar but slightly higher results (I could probably tweak its parameters). It doesn’t fully handle the interplay between running/cycling loads, so it’s better for pure cyclists. It has several great Garmin CIQ app, including the recent Magic Buckets, which is worth a look.

More: Baron BioSys – Xert