CTL and ATL

What it is

CTL (chronic training load) is an exponentially weighted average of your daily training load over about 42 days. It’s a rough picture of the training you’ve been absorbing over the past six weeks or so. ATL (acute training load) is the same calculation over about 7 days, so it rises and falls much faster.

intervals.icu shows these as Fitness and Fatigue (the CTL/ATL names are trademarked by TrainingPeaks). The 42-day and 7-day time constants are defaults and can be changed. The idea comes from Banister’s fitness–fatigue model, which treats each session as adding both fitness and fatigue, with fatigue fading faster.

Why it matters for training decisions

CTL climbing steadily over weeks means you’re consistently training more. ATL tells you how much you’ve loaded yourself recently. The gap between them is form, or TSB. Seeing them together helps you plan build weeks, recovery weeks and tapers without guessing.

Limits

• CTL isn’t fitness in the physiological sense. It measures how much training you’ve done, not how well you’ve adapted. It can rise while performance stalls.
• It’s only as good as the load numbers fed into it, and different load methods give different CTLs.
• Charts need about six weeks of consistent data before CTL means much, because of the 42-day time constant.
• It can’t see sleep, illness, life stress or a niggling injury.

How P247 uses it

P247’s weekly report checks whether your training load rose faster than your recovery.

This glossary is general information to help you understand your training data. It isn't medical advice, diagnosis or treatment. For injury, illness, medication, blood test results or any health concern, speak to a qualified health professional.

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