Training Stress Score (TSS)
Training Stress Score (TSS) is a single number for how much stress a workout puts on your body, combining duration and intensity relative to your threshold. One hour at exactly your FTP or threshold pace scores 100. The name is a TrainingPeaks trademark; other tools use similar load scores.
How TSS is calculated
For cycling with power: TSS = (duration in seconds × normalized power × intensity factor) ÷ (FTP × 3600) × 100, where intensity factor is normalized power divided by FTP. Running and swimming versions use pace relative to threshold pace or CSS, and heart-rate versions exist when there's no power or pace.
How to use it
TSS lets you compare a long easy ride with a short hard one. Summed by day, it feeds the fitness, fatigue and form model (CTL, ATL and TSB). As a rough guide, a single session under 150 TSS is usually recovered from by the next day, while 300 or more can take several days.