The science library Training load & form

Training load, fitness, fatigue and form

What TSS, CTL, ATL and TSB measure, and how to read the curve without over-reading it.

One number for one workout

A two-hour easy ride and a 40-minute interval session are hard to compare by feel. Training Stress Score (TSS) puts them on one scale by combining how long you went with how hard you went, relative to your own threshold ¹.

When a ride has no power data, load can still be estimated from heart rate, or from a session rating of perceived exertion multiplied by duration. Session-RPE tracks objective load closely enough to drive the same curves, which is why a run logged with nothing but a watch and an honest effort score still counts ².

Three curves from one input

Feed daily load into a model that decays over time and you get the fitness–fatigue picture first described by Banister and colleagues ³. Every hard day adds to both a slow-moving trace and a fast-moving one, and the two are subtracted to give the third.

Fitness (CTL) is your load averaged over roughly six weeks. It moves slowly, and it is what you have actually built.
Fatigue (ATL) is the same load averaged over about a week. It spikes after a hard block and fades within days.
Form (TSB) is yesterday's fitness minus yesterday's fatigue. Positive means fresh, negative means loaded.

The useful part is the asymmetry. Fatigue clears far faster than fitness fades, so cutting volume for a week or two drops one line quickly while the other barely moves. That gap is what a taper is engineering ⁴.

Why the rate of change matters more than the level

High chronic load, built gradually, is associated with lower injury risk rather than higher. What raises risk is a sharp jump: a week that badly outruns the previous month. The ratio between recent load and chronic load is the standard way to catch that ⁵ ⁶.

Calculating that ratio with exponentially weighted averages, so the last few days count for more than the days before them, flags spikes earlier than a flat rolling average does ⁷.

Tapering

Across the taper literature, the pattern that produces the largest performance gain is a progressive cut in volume of roughly 40–60% over about two weeks, with intensity held and frequency only slightly reduced ⁸.

Cutting the hard sessions instead of the volume is the common mistake. Intensity is what preserves fitness while fatigue drains away.

What the numbers do not know

TSS treats a steady tempo hour and a shredded interval hour as roughly equivalent, and it says nothing about how a session was distributed across the week. Concentrating hard work into focused blocks produces different adaptations than smearing the same total evenly, even where the weekly load is identical ⁹.

The curve does not know you slept five hours or that work is on fire.
It cannot see muscle damage from a hilly race, so a low-TSS descent-heavy day can hurt more than the number suggests.
A single day is noise. The direction over two or three weeks is the signal.

References

Numbered in order of first appearance. Every link goes to the original work.