Training Stress Score

Training Stress Score (TSS) quantifies session load from duration and intensity relative to threshold power; 60 minutes at threshold equals 100 points.

Definition

Training Stress Score (TSS) is a metric that calculates the load of a training session from its duration and its intensity relative to the individual threshold. It was developed by Andrew Coggan and is used by TrainingPeaks as the standard. One hour at 100 percent of Functional Threshold Power (FTP) yields 100 points by definition.

TSS = (duration_s × NP × IF) / (FTP × 3600) × 100
IF  = NP / FTP

NP is Normalized Power, IF the Intensity Factor. Running, swimming and heart-rate variants use analogous formulas with pace or heart rate as the control metric.

Why it matters

TSS makes sessions comparable that differ in distance, duration and sport. A 90-minute endurance ride and a 30-minute interval workout can be placed on the same scale, summed and planned. Weekly load and the derived values CTL, ATL and TSB are built on this foundation.

The value is only as objective as the threshold behind it, however. An FTP estimated from a 20-minute field test can be off by 5 to 10 percent; because the intensity factor enters the formula squared, that error is amplified in the TSS.

Session type	Duration	IF (typical)	TSS (typical)
Active recovery	60 min	0.55	30
Endurance (zone 2)	120 min	0.70	100
Tempo / sweet spot	90 min	0.85	110
Threshold intervals (e.g. 3 × 12 min)	75 min	0.90	100
VO2max intervals (e.g. 5 × 4 min)	60 min	0.95	90
Long ride	4 h	0.68	185
Road race / middle-distance triathlon	4–5 h	0.80–0.85	250–330

How obseed measures it

obseed calculates TSS according to the available control metric and sport:

Variant	Control metric	Sport
TSS	Power (watts)	Cycling
rTSS	Pace (min/km)	Running, walking, hiking
sTSS	Pace (min/100 m)	Swimming
hrTSS	Heart rate (bpm)	Strength and any sport without performance data

This requires current performance benchmarks for every sport you actively train. If they are missing, obseed falls back to hrTSS, which is considerably less precise.

As the reference, obseed prefers thresholds measured in metabolic testing (VT2 or lactate threshold) over FTP estimates from field tests. Only a measured threshold says something about actual energy supply; a field test delivers at best an approximation that shifts with daily form and pacing.

Example

A cyclist with an FTP of 260 W completes a two-hour ride with a Normalized Power of 195 W.

IF  = 195 / 260 = 0.75
TSS = (7200 × 195 × 0.75) / (260 × 3600) × 100 = 112.5

The session yields roughly 113 TSS. If the true FTP were 245 W, the IF would be 0.80 and the TSS around 127: a 6 percent deviation in the threshold shifts the load value by 13 percent.

References

Allen, H., Coggan, A.: Training and Racing with a Power Meter. VeloPress.