CTL (Chronic Training Load)
A rolling, weighted average of your daily training load over roughly six weeks: the closest thing to a single 'fitness' number.
Chronic Training Load (CTL) is a rolling average of your daily Training Stress Score, weighted so recent training counts more than training from a month ago, but nothing falls off a cliff overnight. It's usually calculated with roughly a 42-day time constant, which makes it an exponentially weighted moving average, not a flat one. It's the "fitness" line on the Performance Management Chart model that TrainingPeaks and similar tools popularized.
Why it's a proxy, not a direct measurement
Nothing about CTL is measured directly from your body. It's entirely derived from the training you logged. A rising CTL generally tracks with rising aerobic fitness, because sustained training load and fitness gains are correlated. But CTL itself is bookkeeping, not a physiological test. Two athletes with identical CTL numbers can have meaningfully different actual fitness if their training composition differed, say all easy volume versus a mix that includes real intensity.
The risk in growing it too fast
The most cited caution around CTL is the rate of increase, not the number itself. Ramping CTL aggressively, jumping several points per week rather than climbing gradually, is one of the more consistent predictors of injury and non-functional overreaching in the sports-science literature on training load. That's part of why a well-built plan increases load in controlled steps rather than maximizing how fast CTL climbs. A plan optimizing purely for the highest CTL as fast as possible is optimizing for the wrong thing.
What it doesn't tell you
CTL says nothing about how fresh you are right now. That's the job of its short-term counterpart, Acute Training Load (ATL). A high CTL built up over months can sit alongside a body that's currently fatigued from a hard recent block. Reading CTL alone, without ATL, misses exactly the tension a taper exists to resolve.