CTL (Chronic Training Load)

CTL is the 42-day exponentially-weighted average of your daily TSS. It represents your fitness — the accumulated effect of consistent training. Rising CTL means you're getting fitter; falling CTL means you're detraining. It moves slowly by design.

CTL_today = CTL_yesterday × e^(−1/42) + TSS_today × (1 − e^(−1/42))
    (EWMA with a 42-day time constant)

The 42-day time constant is Coggan's adaptation of the fitness side of Banister's original fitness-fatigue framework (whose fitted fitness time constant was longer — on the order of 50 days). At τ = 42, roughly 95% of CTL's response to a training stimulus comes from the last ~120 days — so your CTL today is a real reflection of your last 3–4 months of training, not just last week.

CTL in isolation isn't a performance predictor — plenty of high-CTL athletes race flat. What matters is CTL combined with ATL (fatigue) and TSB (balance), plus the rate of CTL change. A sustained CTL ramp above ~8/week over multiple weeks is a common overreaching warning sign, though the safe ceiling is highly individual — most riders tolerate roughly 5–8/week depending on age, training history, and current fitness.