Durability Score – Knowledgebase

The Durability Score measures how much an athlete’s Fitness Signature (TP, HIE, PP) declines during long rides, reflecting resilience to prolonged fatigue. Expressed as a percentage, lower scores mean better durability. Accurate assessment requires rides over 2–3 hours with maximal late efforts, and durability is distinct from overall fitness.

Definition

Durability Score measures how much your Fitness Signature declines during a long activity.

While your Fitness Signature (TP, HIE, and PP) is normally treated as stable during a ride, Durability modeling evaluates how those values change over time due to prolonged fatigue.

In simple terms:

Durability Score reflects how well your fitness “holds up” after hours of riding.

What It Represents

Durability describes a type of fatigue that:

Builds gradually over long durations
Does not meaningfully recover during the ride
Requires full rest to restore

This long-duration fatigue is often associated with:

Glycogen depletion
Muscular fatigue
Accumulated metabolic stress

An athlete with strong durability can still produce high power late in a ride or race.

Understanding the Score

Durability Score is expressed as a percentage decline in your Fitness Signature.

0% → No meaningful change in your signature during the ride
100% (theoretical) → HIE fully depleted and Threshold Power has decayed to Lower Threshold Power (LTP)

Lower scores indicate better durability. Typical durability ranges:

~15%+ → Less trained
~10% → Well-trained club athlete
~5% → Highly trained / elite
<2% → Professional-level durability

Athletes with stronger durability can outperform riders with higher raw fitness but weaker late-race resilience.

How It Works

Xert Durability Score — Durability Assessment Example (6.5-hour ride). After assessing durability, notice how MPA responds more sharply to hard efforts late in the ride. This reflects accumulated long-duration fatigue - as your Fitness Signature declines, high-intensity efforts cause MPA to drop more quickly.

To assess durability:

Select a ride longer than 2–3 hours
Ensure you performed a maximal effort late in the ride
Click Assess Durability

Xert then:

Calculates a durability metric throughout the activity
Models how TP and HIE decline over time
Recalculates MPA using the adjusted signature
Displays how your signature evolved during the ride

After assessing durability, you can mouse over the MPA chart to see how your signature changed over the course of your activity.

Getting a Valid Result

A pure endurance ride without a maximal effort will not produce meaningful durability results. For accurate durability assessment, use the following steps:

Use rides longer than 2–3 hours
Include a true maximal effort late in the activity
Ensure your starting Fitness Signature was accurate

Note: If a Breakthrough occurred during the ride, durability assessment may not be valid.

Fueling Considerations

If using Carbs Used as your Durability Metric:

You may input Carbs Replenished
Reassess durability after entering fueling data

This allows you to evaluate how nutrition strategies impact late-ride performance.

Where You’ll See It

At the bottom of the Activity Details page
- Via the Assess Durability button
Reflected in updated MPA and signature charts

Common Misunderstandings

Durability is not the same as fitness. You can have a high Fitness Signature but weak durability.
Easy long rides do not assess durability. A maximal effort while fatigued is required.

Related Terms