Also known as: ATL, Acute Training Load, Fatigue score
Fatigue is a rolling average of your daily training stress over roughly one week, weighted toward the most recent days. It moves fast, and it tracks how tired your recent training has left you. Coaching software usually calls it ATL, for Acute Training Load.
Training Load answers "how fit am I". Fatigue answers "how tired am I right now, from what I just did". It reacts quickly. One hard long run or a race can spike it inside a day, and a rest day pulls it back down almost as fast. That short memory is the point: Fatigue models the tiredness that clears in about a week, not the fitness that took months to build.
Fatigue is the fatigue half of Banister's 1975 fitness-fatigue model, computed exactly like Training Load but over a much shorter window. Common coaching-software implementations use a 7-day time constant: today's value = yesterday's value + (today's score − yesterday's value) / 7. The short window is what makes Fatigue sensitive to your last few sessions instead of your last six weeks.
Like Training Load, Fatigue has no universal healthy number, because it scales with however training stress is scored for your sport. What means something is Fatigue relative to Training Load. Fatigue sitting well above Training Load for a stretch describes a hard block or a race. Fatigue sitting persistently below it describes a taper, or an easier period.
Fatigue is what turns Training Load from a static fitness score into a decision tool. Subtract one from the other and you get Form, the training balance, which is the number coaches actually use to judge whether an athlete is fresh enough to race or hard-train, or tired enough that tomorrow should be easy. A large gap, with Fatigue spiking far above Training Load, is the clearest early warning of non-functional overreaching.
One big Fatigue spike is not a problem by itself, though plenty of runners read it as one. A controlled spike after a hard block, followed by recovery, is how adaptation works. Watch the pattern instead. Fatigue that stays high week after week, without the recovery days that let it fall, is the load signature most associated with overtraining syndrome in the sports-science literature.
A Fatigue number on its own tells you little. It has to be read against Training Load and against its own recent trend, daily, before it can shape what you do tomorrow.
Recomputing that by hand after every run is the first thing to go in a busy week, which is usually the week you need it.
Vertical tracks Fatigue next to Training Load and Form automatically, and uses the balance between them, not Fatigue alone, to decide whether tomorrow is hard, easy, or off.
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