Xert Strain Score (XSS) – Knowledgebase

The Xert Strain Score (XSS) quantifies the physiological strain during exercise by considering both intensity and duration, adapting for fatigue and individual fitness. It subdivides strain into Low, High, and Peak components, informing personalized training, recovery, and performance recommendations, and differs from traditional metrics like TSS in calculation and application.

Definition

The Xert Strain Score (XSS) quantifies the physiological strain accumulated during an activity, based on both intensity and duration. One hour at the Threshold Power (TP), when unfatigued, equals 100 XSS.

However, under fatigue, one hour at Threshold Power can be significantly higher than 100 XSS.

What It Represents

XSS measures the stress experienced by your energy systems during training. Unlike traditional load metrics that rely primarily on average power throughout an activity, XSS is calculated using work performed second-by-second, relative to your Maximal Power Available (MPA) and normalized to your individual Fitness Signature.

Because XSS accounts for fatigue, the same power output can produce different amounts of strain depending on your MPA at the time the effort is performed. As MPA decreases, you are operating closer to your physiological limit - and therefore accumulating more strain for the same workload.

A simple example is a 20-minute FTP test. Even if you pace the effort perfectly, the final minute feels significantly harder than the first. Traditional training scores would treat each minute of that effort equally. Xert, however, assigns more strain to the final minute because you are riding much closer to your limit at that point, even though your power output is unchanged.

XSS Subdivisions: Low, High & Peak

XSS is subdivided into three components based on which energy system(s) experienced the strain:

XLSS (Xert Low Strain Score): strain accumulated through the Threshold Power system.
XHSS (Xert High Strain Score): strain accumulated through the High-Intensity Energy system.
XPSS (Xert Peak Strain Score): strain accumulated through the Peak Power system.

This breakdown allows Xert to analyze how strain was distributed across your three energy systems and powers features such as Magic Buckets.

This makes XSS adaptive and highly individualized to your training and your goals.

How Xert Uses It

XSS drives many elements in Xert related to training & performance:

Updates Low, High, and Peak Training Loads
Influences Recovery Loads and Training Status
Determines daily training recommendations for XATA & XFAI
The distribution of XSS across Low, High, & Peak systems determines the Focus & Specificity of your training

Where You’ll See It

You’ll encounter XSS in:

Today Page & Training Recommendations
Fitness Planner
Magic Buckets
Activity Details

Common Misunderstandings

While they are similar conceptually, XSS is not equal to TSS. They are calculated using different methods.
Further, higher XSS is not always better. What's important for you and your training is to ensure that you're placing the appropriate amount of XSS on each system, relative to your current fitness & your unique goals as an athlete. Xert helps you achieve this.

Related Terms