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 + (TSS_today − CTL_yesterday) / 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 (Banister's 1975 paper used 45 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 ramp rate above ~7 CTL/week sustained for multiple weeks is strongly associated with overreaching.