The Xert Strain Score quantifies how much Strain an athlete endures during an activity. By taking Strain, which is the amount of work performed close to MPA, and normalizing it with the athlete’s Fitness Signature, one-hour at Threshold Power equates to 100 XSS. Note that an athlete can experience more than 100 XSS in an hour if the work is performed under short-term fatigue. For more, see Xert Strain Score.