Fitness, fatigue and form

Individual load scores say little on their own. What's interesting is something else: what has built up over months, what hasn't cleared since yesterday, and where you stand right now. So the scores are gathered into three lines — shown on the fitness, fatigue and form chart.

Fitness: what you've banked#

Fitness is a model of the aerobic work accumulated over roughly the last six weeks. It's your foundation: slow to build, slow to fade.

A week on holiday barely dents it. A month without training will drop it noticeably, and that's the truth rather than the platform being fussy.

One important caveat: fitness is not an “athlete level” and not a measure of health. It knows nothing about your technique, your economy or your head. It's a counter of accumulated work — no more than that, but no less either.

Fatigue: what hasn't cleared#

Fatigue is the same accumulated work, but with a short memory of about a week. A hard block lifts it within a couple of days; a few easy days bring it down just as quickly.

Form: where you are today#

Form is the difference between fitness and fatigue. The easiest way to read it:

• Negative — the load hasn't cleared yet. You're in the work.
• Around zero — the familiar state of everyday training.
• Positive — you're rested; this is what being tapered for a race looks like.

Negative form in the middle of a build block isn't a warning, it's a sign that you're genuinely training. Positive form comes for the race, after the taper. Form isn't a goal in itself and isn't a rating of how you feel: it's a planning tool.

In the week's terms and in the miss receipt the number is labelled “fitness” — it is CTL at the start of the next key day, worked out with the same formula on the assumption that the other sessions go to plan. Form is not given as a number there: only the balance zone stands next to it, the same one as on this page. It is a load-model calculation, not a readiness forecast, and it stays hidden while the series is calibrating.

Why the memory is 42 and 7 days#

Both lines are weighted rolling averages: recent days count for more than old ones. Exactly how much more is set by the memory length: 42 days for fitness and 7 days for fatigue.

These numbers are the same for everyone, and that's a deliberate decision. Tuning them to a particular person sounds tempting, but with amateur data such tuning can't be verified: it gives a pretty match with the past and no predictive power at all. We'd rather have one honest model for everyone than a personal one that fools us.

Muscles are counted separately#

Aerobic fatigue and muscular fatigue are different things. After a long run your heart may be ready tomorrow while your legs are not.

So alongside the three lines the platform keeps a separate, fast-moving counter of muscle and impact load — from running, jumping and strength work, with a memory of a few days. It's the thing that stops impact work being put on heavy legs, even when the form chart looks perfectly decent. How that feeds into the decision about your day is in Readiness.

Why history isn't rewritten after the fact#

When your threshold is updated, the new value applies going forward. Past sessions keep the scores they were calculated with.

That's deliberate. If history were recalculated at every update, the chart would be redrawn from end to end and you'd no longer be able to tell what actually happened — you'd be looking at the latest version of our beliefs about your training, not the training itself. We only recalculate everything when we're fixing errors in the source data, and we always say so.

How to read the chart#

Fitness rising, form deep in the negative. A build block going as it should. Watch your readiness and how you feel — they'll tell you whether it's time to ease off.

Fitness flat, form around zero. Maintenance mode. Fine in the off-season and between goals, but growth will need a plan with progression.

Fitness falling, form positive. Either a taper — in which case all is well — or you're training less than you think.

Fitness has dropped even though you were training. Usually it isn't you: some sessions didn't make it across from an external service, or their load didn't get calculated. Open the cards of your recent sessions — if the score is empty, add an effort rating and it will appear.