Shindo The three numbers#
Every endurance platform that models training load computes the same three things from your daily training stress. The names differ; the maths does not.
Fitness — chronic training load, or CTL. A 42-day exponentially weighted average of your daily training stress. It rises slowly when you train consistently and falls slowly when you stop. Read it as how much work you have adapted to.
Fatigue — acute training load, or ATL. The same average over 7 days. It rises within days of a hard block and falls within days of rest.
Form — training stress balance, or TSB. Fitness minus fatigue. Positive means you are carrying more adaptation than tiredness — you are fresh. Negative means the reverse.
The 42 and the 7 are conventions, shared across the major platforms, which is what makes a fitness number roughly portable between them.
What the numbers look like in practice#
Absolute values depend on your sport and on how the platform scales training stress, so treat these as orientation rather than as targets:
| Form (TSB) | What it usually means |
|---|---|
| Above +20 | Very fresh. Good for racing, and if it persists for weeks you are detraining |
| +5 to +20 | Race-ready. This is where you want to be on the start line |
| −10 to +5 | Neutral. Normal maintenance training |
| −30 to −10 | Productive training. Where most of a build block lives |
| Below −30 | Deep fatigue. Sustainable briefly, and where injury and illness risk climbs |
For fitness, what matters is the trajectory, not the value. A recreational runner might sit at 40 and a professional at 130, and neither number says anything about the other person. Your own curve going up over a block means the block is working.
The single most useful thing the chart does#
It separates two things that feel identical from the inside: being unfit and being tired.
An athlete four weeks into a build feels slow, heavy and discouraged. The chart shows fitness climbing steadily and form at −25, and the correct interpretation is “this is working, and you are buried under the work that is making it happen”. Without the chart the same athlete concludes they are getting worse and either trains harder — which is how injuries happen — or gives up.
That is genuinely most of the value. The rest is refinement.
Using form to time a race#
The practical application. Fitness is what you have built; form is what you can express on the day. A taper works by letting fatigue fall while fitness — which decays far more slowly — stays largely intact.
The mechanism in numbers: fatigue has a 7-day time constant and fitness a 42-day one. Cut volume for ten days and fatigue collapses while fitness barely moves. Form rises, and you arrive fresh on a base you spent months building.
In practice:
- Aim to arrive at race day with form somewhere around +5 to +20. Higher is not better — a very large positive number usually means you tapered too long or too hard and have started losing fitness and sharpness.
- Cut volume, keep intensity. Volume carries the fatigue; intensity keeps you sharp. A taper of easy jogging only tends to leave people feeling flat.
- Typical tapers run one to three weeks depending on the event, with volume down 40–60%.
- Do not build fitness in the last two weeks. It is too late for that block to produce adaptation, and all you add is fatigue you then cannot shed.
Four mistakes people make reading the chart#
1. Chasing fitness as a score. A rising number becomes the goal, and the athlete starts adding junk volume to keep it climbing. Fitness is a proxy for adaptation, not the thing itself — a hundred easy miles and a well-structured seventy can produce the same number and completely different athletes.
2. Panicking at negative form. Negative form during a build is the point. It only becomes a problem when it goes very deep, stays there for weeks, or is accompanied by signals that you are not absorbing the work.
3. Forgetting the chart cannot see everything. Strength work, life stress, illness, a bad week at work, poor sleep and underfuelling all produce real fatigue that never appears as training stress. An athlete whose numbers look fine and who feels destroyed is usually being told something true by their body and nothing by the chart.
4. Comparing curves with other people. Different platforms, different sports, different thresholds. The comparison is meaningless and the temptation is strong.
The shape of a healthy block#
If you look at nothing else, look at the shape:
- Fitness climbing steadily, at a moderate rate — very roughly 3–5 points a week for most athletes. Faster than that is where overuse injuries live.
- Fatigue oscillating: rising through loading weeks, dropping through recovery weeks.
- Form sawtoothing between about −30 and 0, rising into a recovery week and dropping again in the next loading week.
A flat fatigue line means every week is the same, which is the monotony problem: no real hard days and no real easy days. A fitness line that climbs without interruption for twelve weeks means no recovery weeks were taken, and it usually ends the same way.
Shindo computes all three on the conventional 42-day and 7-day constants and charts them for free, with no subscription. It also caps planned weekly growth at 8% so the fitness line cannot climb faster than your tendons can follow — the methodology explains why.
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