ATL (Acute Training Load)
A rolling average of your daily training load over about a week: a measure of how much fatigue you're carrying right now.
Acute Training Load (ATL) is the short-term counterpart to Chronic Training Load (CTL). It's also a rolling, weighted average of daily Training Stress Score, but over roughly a week instead of roughly six. CTL moves slowly and represents accumulated fitness. ATL moves fast: it climbs quickly during a hard training block and drops quickly once volume or intensity backs off.
What the gap between CTL and ATL means
Subtracting ATL from CTL gives a number sometimes called Training Stress Balance (TSB), or informally "form." A strongly negative TSB means ATL is running well above CTL: recent training has been harder than your body has adapted to yet. That shows up as fatigue, heavy legs, flat power numbers, a session that should feel easy but doesn't. It's not automatically a bad thing during a training block. Some accumulated fatigue is the normal cost of building fitness. It only becomes a problem if it never gets a chance to resolve.
Why this is exactly what a taper addresses
A taper works by deliberately dropping training volume so ATL falls faster than CTL does. CTL decays slowly by design, so a well-timed taper lets fatigue clear while fitness stays mostly intact. That's the whole mechanical point of tapering. It isn't resting because rest feels nice before a race. It's closing the gap between CTL and ATL on purpose, so an athlete reaches the start line with the fitness they built and without the fatigue that was masking it.
The common mistake
Athletes sometimes read a big fatigue dip (a strongly negative TSB) as a sign something's wrong, and panic-rest mid-block. Often it's just the expected state during a hard week. The useful read isn't a single day's ATL. It's the trend, and whether the taper that follows actually gets a chance to resolve it before race day.