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Open the Performance Management Chart of any power-based platform and you find the same three lines. One climbs slowly over months, one jumps up and down with every hard week, and the third swings either side of zero. They are CTL, ATL and TSB, and all three are made from one input: the training load of each day.
The short answer
CTL (fitness) is a slow exponentially weighted average of your daily Training Stress Score and ATL (fatigue) a fast one; TSB (form) is their difference, yesterday's CTL minus yesterday's ATL. This is the Performance Manager model of Allen, Coggan and McGregor, whose conventional time constants are about 42 and 7 days. A negative TSB means you are carrying fatigue, which is normal and intended in a build; a positive one means you are fresh, which is what a taper is for.
It all starts with TSS
Training Stress Score puts one number on a ride by combining its duration with its intensity relative to your threshold. An hour ridden at your FTP scores 100 by definition, an easy hour scores much less, and because intensity enters the formula squared, hard minutes weigh more than easy ones. The definition belongs to the same Performance Manager model, set out in Training and Racing with a Power Meter.
Without a power meter, the load can be estimated from heart rate against your threshold heart rate (LTHR). It is a reasonable stand-in on steady rides and reads high on long ones, because cardiac drift raises heart rate at the same work (Coyle and González-Alonso, 2001).
Three numbers from one input
| Number | Name | What it is |
|---|---|---|
| CTL | Fitness | The slow weighted average of daily TSS, conventionally about 42 days. Slow: months of base and build raise it, and one week barely moves it. |
| ATL | Fatigue | The fast weighted average of the same TSS, conventionally about 7 days. Fast: it climbs with every hard block and clears within days of easing off. |
| TSB | Form | Yesterday's CTL minus yesterday's ATL: how much of your fitness you can use today. |
How the averages are calculated
Both averages are exponentially weighted, which means each day pulls the number a fixed fraction of the way toward that day's TSS. Recent days count most and old ones fade gradually instead of dropping out on a fixed date. Here is one illustrative week for a rider who starts it at a CTL and an ATL of 50:
| Day | TSS | CTL | ATL | Form next morning |
|---|---|---|---|---|
| Mon | 0 | 48.8 | 42.9 | +5.9 |
| Tue | 90 | 49.8 | 49.6 | +0.2 |
| Wed | 40 | 49.6 | 48.2 | +1.4 |
| Thu | 100 | 50.8 | 55.6 | −4.8 |
| Fri | 0 | 49.5 | 47.7 | +1.8 |
| Sat | 150 | 51.9 | 62.3 | −10.4 |
| Sun | 60 | 52.1 | 62.0 | −9.9 |
Illustrative, computed with the published model's conventional constants, not Summit's internals. Form (TSB) is the day's CTL minus its ATL, read the next morning.
Read the columns rather than the cells. Fitness ends the week two points up after 440 TSS, because 440 is above the roughly 350 a week (CTL × 7) that would have held it level, the rule of thumb the base, build, peak piece works with. Fatigue swings by almost twenty points within the same seven days, and form follows it: positive after the rest days, ten below zero after the weekend.
How to read them
Fitness rising slowly
- You are training above your recent average
- Raise it gradually: the ramp rate matters
Form negative in a build
- Normal and intended: you are fit and tired at once
- The fatigue is the price of the fitness you are adding
Form deeply negative for weeks
- Fatigue is outrunning recovery
- A recovery week is overdue, before the next hard session stops landing
Form positive before an event
- The point of a taper: fatigue falls faster than fitness
- How far positive, and for how long, is in the taper and peak piece
Two habits make the chart more useful than any single value on it. Compare each number with your own history rather than with another rider's, because a CTL reflects how much you have trained, not how good you are. And read the shape over a week or more, because the daily values move with every ride and the trend is what carries the information.
What the model cannot tell you
- It counts load, not adaptation. CTL rises whether the training is making you fitter or digging a hole.
- It knows nothing about your night. Sleep, illness and stress never enter TSS, which is why recovery is read separately, from HRV, overnight heart rate and sleep. That side is covered in How HRV should change today's ride.
- It needs history. From an empty start the slow average takes months of rides to reach your real level, so early numbers understate your fitness.
- 42 and 7 are conventions. They are the model's standard time constants, not measurements of your physiology.
How Summit uses them
Summit uses the same Performance Manager model, applied to your own rides from Strava or a Hammerhead Karoo. Small differences from other tools are normal, because each tool computes TSS from its own ride data. Summit does not claim a better CTL, only a better use of it. Rides without power count through heart-rate load, and a heart-rate rider's Training tab shows the chart built from it; eFTP, critical power, W′ and the power curve need power data.
On Pro you get the Performance Management Chart, a 7-day readout of fitness, fatigue and form and where you sit today, with notifications for a new eFTP and for peak form. On Elite the three numbers stop being a chart and start steering the plan. The season is worked back from your event date, with a phase, a fitness trajectory and a band for form for every week. The taper lets fatigue fall faster than fitness so that form rises into the race. And day to day, on an otherwise good recovery reading, a one-off dip only softens the session when your form confirms the fatigue.
The product side, with what each plan includes, is on the training load page, and the prices with their caveat are on the pricing page.
Questions
What is a good CTL for a cyclist?
There is no universal good value. CTL reflects how much you have been training, not how good you are, and it depends on your threshold, your hours and your event. Compare it with your own history and with what your event asks of you, and raise it gradually.
Is a negative TSB bad?
No. A negative TSB means you carry more recent fatigue than fitness, which is normal and intended during a build. It becomes a problem when it stays deeply negative for weeks without a recovery week, or when it is still negative on the day you want to perform.
Why 42 and 7 days?
They are the conventional time constants of the Performance Manager model: one slow enough to stand for months of training, one fast enough to follow the last week. They are conventions, not measurements of your physiology.
Does Summit calculate CTL the same way as other apps?
Summit uses the same Performance Manager model, applied to your own rides from Strava or a Hammerhead Karoo. Small differences from other tools are normal, because each tool computes TSS from its own ride data. Summit does not claim a better CTL, only a better use of it.
Do I need a power meter for CTL, ATL and TSB?
No. A ride without power still counts, because its load is computed from heart rate, and a heart-rate rider's Training tab shows the chart built from it. eFTP, critical power, W′ and the power curve do need power data.
Sources, in the app's own sources sheet
- Allen, Coggan & McGregor (2019), Training and Racing with a Power Meter, 3rd ed.: the Performance Manager model the load engine implements, TSS, CTL, ATL and TSB.
- Coyle & González-Alonso (2001): cardiovascular drift, why heart-rate load reads high on long rides.
See your own three lines.
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