TriPeak

Training Stress Score, and what it can and cannot tell you

TSS puts a single number on how much a session took out of you, so that an hour of intervals and four hours of endurance can be compared, added up and planned against.

The definition

One hour at threshold is one hundred points. Everything else is scaled from that, by how long the session was and how hard it was relative to your own threshold — which is what makes the number comparable between two athletes of very different ability, and comparable for one athlete only as long as their threshold is current.

Intensity Factor (IF) is the session's intensity as a fraction of threshold: 1.0 is threshold, 0.75 is endurance, 1.05 is above. TSS is then hours multiplied by IF raised to a power, multiplied by a hundred.

Bike

TSS = hours × IF² × 100, where IF = normalised power ÷ FTP

Run and swim

TSS = hours × IF³ × 100, where IF = your speed ÷ threshold speed

One hour at threshold

100 points, in every sport, by definition

Why the exponent is not the same in every sport

On the bike, intensity is measured as power, and power is already the rate at which work is being done — squaring it reflects how disproportionately the cost rises near threshold.

For running and swimming there is usually no power meter, so intensity is measured as speed. Speed is not proportional to effort: going five percent faster costs far more than five percent more. Cubing the speed ratio approximates that, which is why the same IF produces a different TSS depending on which sport it came from.

This is also why an intensity in TriPeak is always written as a fraction of threshold in the direction of effort. 105% means harder than threshold in all three sports — on the bike, 105% of FTP; running or swimming, five percent faster than threshold speed, not five percent slower per kilometre.

What the numbers mean in practice

Under 50

Recovery, or a short easy session. You could do it again tomorrow.

50–100

A normal weekday session. Fully recovered the next day.

100–200

A hard or long session. Some residual fatigue into the next day.

200–300

A long ride or a big brick. Expect two days.

Over 300

Race day, or a training day that should be planned for like one.

Where the number stops being trustworthy

• A stale threshold makes every score wrong in the same direction. If your FTP has risen and the profile has not, every session reads harder than it was, and the fitness curve inflates.
• Heart-rate-derived TSS lags the effort and drifts with heat, caffeine, altitude and how much you slept. It is the estimate of last resort, used only when there is no power and no reliable pace.
• GPS pace indoors, in a pool, or under trees is not pace. A session whose pace cannot be trusted should not be scored from it.
• TSS counts mechanical and metabolic cost, not neuromuscular damage. A hilly run and a flat ride can score the same and cost very different amounts of recovery.
• It says nothing about whether the session was the right one. A hundred points of the wrong work is still a hundred points.