Fitness, fatigue, form.

CTL — Chronic Training Load (“fitness”)

42-day exponentially weighted moving average of daily TSS. Climbs slowly as you train consistently. A well-trained amateur cyclist sits at CTL 60–90. A cat 2 racer at 90–120. An off-season amateur drops to 30–40.

ATL — Acute Training Load (“fatigue”)

7-day EWMA of TSS. Responds fast. A single 200-TSS ride shifts ATL by 30+ points the next day. Recovers in 3–5 days.

TSB — Training Stress Balance (“form”)

CTL minus ATL. Positive = fresh, negative = loaded. This is the number you check before a race.

TSB > +25 — Stale / overtapered

You’re too fresh. If this is mid-season, you’re undertraining. If this is race week, you’ve cut too much.

TSB +5 to +25 — Fresh (good for races)

Peak performance window. Most athletes race fastest at TSB around +15. Cut volume, keep some intensity.

TSB −5 to +5 — Fit (productive training)

The sweet spot for building. You’re absorbing the load and adapting. CTL should be climbing slowly.

TSB −10 to −30 — Productively overloaded

Short-term block of hard training. Fine for 2–3 weeks if you follow with a deload. Don’t race in this band.

TSB < −30 for 14+ days — Overreached

Injury and illness risk climbs sharply. Our rule engine flags this as NEGATIVE_TSB_PROLONGED and will force a recovery block.

Experienced amateur: CTL should climb no faster than 3.5 points per week. Beginner or returning: 2.5 per week. Faster than 5 per week is explicit injury risk and shouldn’t be prescribed.

Real ramps are rarely smooth — you’ll see 2-week build steps of 4–5 points, then a deload week where CTL plateaus or drops by 2. That’s healthy.

Both CTL and ATL are standard single-pole EWMAs with time constants 42 and 7 days:

CTL(today) = CTL(yesterday) × e−1/42 + TSS_today × (1 − e−1/42)

To give CTL a stable starting point we run a 126-day warmup window (5 × time-constant rule) before the chart you see. If you’ve been on Strava 6+ months, your CTL is real. If you’ve just connected, we show you the last 60 days and flag lower confidence in the review.