3  Training Load (CTL/TSS)

3.1 Concept

Training Stress Score (TSS) and similar metrics estimate workout load from duration and intensity relative to threshold. Chronic Training Load (CTL) averages daily scores, usually with a 42-day weighting favouring recent training. Apps often call it “fitness,” but it measures accumulated load.

Acute Training Load (ATL) typically covers seven days. Training Stress Balance (TSB), or “form,” compares chronic and acute load to estimate freshness. These models cannot measure recovery directly or override persistent soreness, poor sleep or unusually difficult easy runs.

3.2 Application & Correlation with Performance

Copeland tracks sustained load alongside race results. Use that comparison yourself: does a manageable training increase eventually accompany better performance? CTL alone cannot establish this, and scores are not directly comparable between runners or calculation methods.

Progress can occur at steady load. Follow the progression guidance, reviewing recovery and performance over several weeks, rather than adding sessions to reach an app’s coloured zone.

Different interval formats can produce similar scores without equal recovery demands. Faster running, longer uninterrupted efforts and unfamiliar terrain may feel different the next day. Use that response when judging whether a substitution works.

3.3 Tracking Tools

Intervals.icu, TrainingPeaks and Runalyze offer load tracking. Choose a consistent metric and update its threshold settings as fitness changes. Pace-based estimates suit comparable routes and conditions, but heat, wind and hills can make them understate effort. HR and power estimates have their own limitations. Read trends alongside running time, perceived effort and race results.