What is training stress score (TSS)?
A single number combining how long you trained and how hard, so that sessions of different types can be added together into a weekly load.
Training load models need one currency. Training stress score provides it by weighting duration by intensity, so that an hour of easy running and an hour of intervals do not count the same. The weighting is deliberately non-linear: higher zones contribute disproportionately more stress per minute, which reflects both the metabolic cost and the recovery demand.
In this site's model, load is accumulated per kilometre by zone, with easy running weighted lowest, threshold running roughly twice as much, and interval work around four and a half times as much. Those weights are calibrated rather than measured, which is worth knowing: they produce sensible relative orderings, not physical units.
The value of a load score is entirely comparative. There is no meaningful absolute interpretation of "400 units this week" — the number matters only against your own recent history, which is precisely what ACWR and CTL do with it.
The classic failure mode is a load model that under-weights intensity. If a hard interval session scores similarly to an easy run of the same duration, every derived metric downstream — ACWR, CTL, TSB — will quietly under-report risk in weeks with a lot of quality.
Key numbers
| Easy running | 0.65 units/km | Zone 1 |
|---|---|---|
| Threshold | 1.5 units/km | Zone 2 |
| Intervals | 3.0 units/km | Zone 3 |
Work it out for yourself
Training adaptation map — See how each workout type contributes to weekly load and to injury risk.
Frequently asked questions
Is TSS comparable between runners?
Not really. Load scores are anchored to individual thresholds and calibrated weightings, so comparing your weekly total with someone else's says very little. Comparing this week with your own last four weeks says a lot.
How much weekly load is too much?
There is no universal number. The useful question is how this week compares with your recent four-week average — which is what the acute:chronic workload ratio measures, and why it is more informative than any raw total.
Sources
- Foster et al. (2001). A new approach to monitoring exercise training. Journal of Strength and Conditioning Research.
- Coggan & Allen (2010). Training and Racing with a Power Meter.
Related explainers
Glossary: TSS (Training Stress Score)