TSS = (seconds × NP × IF) / (FTP × 3600) × 100, where IF is Intensity Factor (NP/FTP). It provides a common currency for comparing different sessions: a 2-hour easy ride might be 80 TSS while a 1-hour threshold session is 75 TSS. Weekly TSS targets: recreational (200-300), competitive amateur (400-600), elite (700-1000+). TSS measures load but not stress — Prof. Seiler emphasises that the same TSS from different intensity distributions produces different adaptations.