Most training apps draw three lines over your season: fitness, fatigue and form, also called CTL, ATL and TSB. They come from one of the oldest models in sports science, and they answer a useful question: is the load of the last weeks building you up, wearing you down, or leaving you fresh for a race? This guide explains what each line means, how it's worked out, which numbers to aim for, and where the model stops being reliable.
In short. Fitness is your average daily training load over about six weeks; fatigue is the same over about one week; form is fitness minus fatigue. Building fitness means living with negative form. Before an important race, a taper lifts form to around +5 to +25. The lines are a guide to load, not a measure of how fast you are.
Where the numbers come from
Every session gets a load score, usually TSS (training stress score). It combines how long you trained and how hard compared with your threshold: an hour at threshold scores about 100, an hour easy around 40 to 60. Power, pace or heart rate can all be used to work it out, depending on the sport and what your watch records.
The three lines are built from those daily scores. The idea goes back to the 1970s, when researchers modelled performance as the balance between two effects of each training session: a slow, lasting gain in fitness and a quick, short-lived build-up of fatigue. Today's apps use a simplified version of that impulse-response model.
Fitness (CTL)
Chronic training load is a rolling average of your daily TSS, weighted so that recent days count a little more, over roughly the last 42 days. It rises slowly when you train consistently and falls slowly when you stop. A CTL of 50 means you've been averaging about 50 TSS a day, around 350 a week.
It's the best single number for how much training your body is used to. It is not a measure of how fast you are: two runners with the same CTL can be very different athletes.
Fatigue (ATL)
Acute training load is the same kind of average over about 7 days. It reacts quickly: a big weekend sends it up, two easy days bring it down. It tracks how much you've done lately, which is roughly how tired you should expect to be.
Form (TSB)
Training stress balance is fitness minus fatigue (usually yesterday's values, so today's session doesn't count against today's form). When fatigue is above fitness, form is negative: you're doing more than you're used to. When fatigue drops below fitness, form turns positive: you're fresher than your training level.
What numbers to aim for
These are rules of thumb used by many coaches. Your own history is a better guide, so note where your form was before your best races.
| Form (TSB) | What it usually means |
|---|---|
| +15 to +25 | Fresh. Typical for an important race after a full taper. |
| +5 to +15 | Rested. Good for a B race, or an A race if you don't like feeling too fresh. |
| −10 to +5 | Neutral: maintaining, or between blocks. |
| −10 to −30 | Building. Normal in a hard block; this is where fitness grows. |
| Below −30 | Very tired. Fine for a planned overload week, not for weeks in a row. |
For long ultras, many athletes race well with form a little lower than for short races: they keep more volume late in the taper and arrive around +5 to +15 rather than +25.
How fast fitness should rise
The weekly change in CTL is called the ramp rate. A rise of about 3 to 6 points a week is a common, sustainable range; much more than that for several weeks in a row is where illness and injuries tend to appear. Easier weeks will flatten the line or dip it slightly, which is intended.
If the line is flat for months, the training isn't progressing. If it climbs steeply, check how you sleep and how your legs feel before adding more. Our guide to planning an ultra-trail season covers how blocks and easier weeks shape this line.
Where the model falls short
- A score is only as good as its inputs. Wrong zones or an outdated threshold make every TSS wrong. Heart-rate-based scores also drift with heat, caffeine and fatigue.
- Not all load is the same. 100 TSS of easy running and 100 TSS of hard intervals count the same, but they don't tire you the same way. A long descent costs your legs far more than its score says.
- Strength training is hard to score. Many tools leave it out, so fatigue looks lower than it is in weeks with heavy strength work.
- Life isn't in it. Poor sleep, work stress or an illness change how you absorb training, and the lines can't see them. That's why recovery markers like heart-rate variability are worth reading alongside form.
- It describes load, not performance. A higher CTL usually helps, but it doesn't predict a race time on its own.
An example: one block and a taper
Take a runner whose fitness sits at 45 at the start of a four-week block. The first three weeks are bigger than usual, around 500 TSS each. Fatigue climbs to about 70, form drops to around −15, and fitness rises by three or four points a week to about 55. The runner feels heavy-legged but recovers between sessions: that's the block working.
The fourth week is easier, about 280 TSS. Fatigue falls back to about 50, form comes up to around zero, and fitness dips by a couple of points. The next block starts from a higher base, and the pattern repeats.
Before the race, two weeks of taper cut the load by half while keeping a few short, fast efforts. Fatigue drops much faster than fitness: fitness slips from, say, 62 to 55, while fatigue falls from 70 to 40. Form ends around +15 on race day. Losing a few points of fitness is the price of arriving fresh, and it's worth paying.
Notice what the numbers don't say: whether the long runs had enough climbing, whether the runner practised eating, whether the descents were trained. Load is one part of preparation, not all of it.
How to use the three lines
- Watch fitness for the big picture: is it rising block by block toward your race?
- Use fatigue to spot spikes: a jump of 20 or 30 in a few days is worth a second look.
- Use form to time things: hard blocks below zero, the taper bringing it up for race day.
- Check the lines against how you feel and your recovery readings. When they disagree, trust your body and look for the reason.
See your fitness, fatigue and form in MyTrainingDesk
MyTrainingDesk works out each session's load from your watch's files and draws your fitness, fatigue and form over the season, next to your heart-rate variability, resting heart rate and sleep. Its coach reads them when it plans your week.
Read next
Sources
- Banister EW, Calvert TW, Savage MV, Bach T. A systems model of training for athletic performance. Australian Journal of Sports Medicine, 1975; 7: 57–61.
- Allen H, Coggan A. Training and Racing with a Power Meter. 2nd edition. VeloPress, 2010.