Grundlagen

Understanding Training Load — TSS, CTL, ATL and TSB

What TSS measures, how fitness (CTL), fatigue (ATL) and form (TSB) come out of it — and why negative form in the middle of a build block is exactly right.

Understanding Training Load

Two athletes each run for an hour. One easy, one at threshold. Both "trained for an hour" — the load they absorbed was entirely different. Duration alone is therefore not a usable measure, and kilometres are worse still, because they make nothing comparable across disciplines.

What is missing is a number that combines duration and intensity and means the same thing for swimming, cycling and running.

TSS: the load of one session

The Training Stress Score, developed by Andrew Coggan and Joe Friel, does exactly that. Its definition is simple:

One hour exactly at threshold equals 100 TSS.

Everything else follows. The formula is:

TSS = duration (hours) × IF² × 100

The intensity factor IF is the ratio of the intensity you produced to your own threshold. On the bike that is power against FTP, running it is threshold pace against actual pace, swimming it is CSS against actual pace per 100 metres.

Two examples:

Two hours on the bike at IF 0.80: 2 × 0.64 × 100 = 128 TSS.
One hour of running with a threshold pace of 5:00/km at an actual 5:45/km: IF = 300/345 = 0.87, so 1.0 × 0.87² × 100 = about 76 TSS.

The square on IF is the decisive part. Intensity counts disproportionately: twice the intensity does not cost twice as much, it costs four times as much. That is exactly why an easy long run is more recoverable than its time cost suggests, and a short hard session more expensive.

Without power measurement you can estimate TSS from heart rate (hrTSS) or use Banister's TRIMP. Both are coarser but usable, as long as you do not mix the methods.

From many sessions to a trend

A single number per session says nothing about form yet. It gets interesting when you weight sessions over time — twice, with different memory.

CTL — Chronic Training Load, your fitness. An exponentially weighted moving average of daily TSS over a 42-day time constant. CTL rises slowly and falls slowly. It is what people loosely call "season form": the product of weeks and months, not of yesterday.

ATL — Acute Training Load, your fatigue. The same calculation over 7 days. ATL reacts fast: a hard week drives it up, three quiet days bring it back down.

TSB — Training Stress Balance, your form. The difference:

TSB = CTL − ATL

Together these three curves are called the Performance Management Chart.

How to read TSB

The number is less intuitive than it looks, and it gets misread regularly.

Clearly negative (roughly −10 to −30): you are training more than you are currently absorbing. In the middle of a build block this is the normal state and not a warning — that is where adaptation comes from.
Around zero: load and recovery are balanced. Typical for recovery weeks and stretches of steady volume.
Clearly positive (roughly +5 to +25): you are fresh. This is exactly what the taper aims at.
Permanently strongly positive: you are fresh and losing fitness. Fine after the season, a sign that too little is happening in the middle of a build.

The most common mistake is chasing positive form all the time. Wanting to feel fresh constantly means training too little to improve.

The ramp rate

More important than the absolute CTL value is how fast it climbs. That weekly increase is called the ramp rate.

As a guide, 3 to 5 CTL points per week suits well-trained athletes. Beginners should stay below it. Progressing much faster over a longer stretch accumulates a fatigue that shows up not in a single session but as a creeping drop in performance, poor sleep and a higher susceptibility to infection.

Where the limits are

These numbers are a model, not a measuring device. Three caveats worth knowing:

A wrong threshold distorts everything. If your stored FTP is too low, every session looks harder than it was — and CTL appears to climb faster. Thresholds want re-testing every six to eight weeks.
The model only knows training load. Lack of sleep, illness, work stress and heat do not appear in it but act on the same body. Resting heart rate and HRV are a sensible complement, not a replacement.
The 42 and 7 day constants are conventions, not individually measured values. They work well enough for most people to show trends; as absolute truth about your own body they do not hold.