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TSS, CTL, ATL and TSB explained simply

Four numbers describe how hard you trained, how fit you are, how tired, and whether you are fresh enough for a hard session. Here is how they arise, with a worked example and honest about the limits.

Updated: October 6, 2026 · 8 min read

TSS: the load of one session

Training Stress Score (TSS) sums up a session in a single number. It accounts for duration and intensity. One hour exactly at your threshold equals 100 points. In cycling the threshold is your FTP, and this is the calculation:

• NP (normalized power) weights hard phases more than the plain average: 30-second rolling average, raised to the fourth power, averaged, fourth root.
• IF (intensity factor) is NP ÷ FTP.

Example: 90 minutes (5400 s), NP 200 W, FTP 250 W. IF is 200 ÷ 250 = 0.8. TSS = 5400 × 200 × 0.8 ÷ (250 × 3600) × 100 = 96.

Running and swimming follow the same principle, using pace instead of power (rTSS, sTSS). Without power or pace, load can be estimated from heart rate; that needs your threshold heart rate and is less precise. How all sports end up on one scale is explained in “Load across sports”.

CTL: fitness

Chronic Training Load (CTL) is an exponentially weighted average of your daily TSS over about 42 days. Recent days count more, older ones less. In simplified form:

Example: CTL 58 and a session of 100 TSS give 58 + (100 − 58) ÷ 42 = 59. Fitness rises slowly and falls slowly. That inertia reflects that adaptation takes weeks.

ATL: fatigue

Acute Training Load (ATL) works the same way but over about 7 days. It reacts quickly to hard days and drops fast after rest days.

Example: ATL 70 and 100 TSS give 70 + 30 ÷ 7 ≈ 74.3.

TSB: form

Training Stress Balance (TSB) is fitness minus fatigue: form = CTL − ATL. With the numbers above: 58 − 70 = −12. You are fatigued but training in the productive range.

Tools differ: some use the same day's values, ZonePit uses the previous day's, i.e. how you start the day.

These are guidelines. How much fatigue you tolerate is individual.

Ramp: how fast does fitness grow?

Ramp is the CTL gain per week. Staying above roughly 5 to 8 points per week for long counts as fast; ZonePit turns the number orange above 8. If the ramp stays high for long, a recovery week makes sense.

Limits of the model

• It is an approximation: sleep, stress, illness and nutrition are not part of it.
• The numbers are only as good as your thresholds (FTP, threshold pace, threshold heart rate). A wrong threshold shifts every value.
• The curves start at zero and build up from your workouts. Fitness only becomes meaningful after a few weeks of history. Where data is missing, the gap stays visible.

Keep reading

Sources

• Hunter Allen, Andrew Coggan: Training and Racing with a Power Meter (TSS, NP, IF, CTL, ATL, TSB).
• E. W. Banister et al. (1975): A systems model of training for athletic performance (basis of the fitness-fatigue model).

ZonePit is a training and wellness app, not a medical device.