Understanding your numbers: intensity, zones and CTL/ATL/TSB

How intensity is decided

The AI weighs your current CTL (fitness), ATL (fatigue), TSB (form), recent sleep and HRV, power/pace zones, upcoming events, and the weekly availability you set. It balances progressive overload with recovery. Targets are based on your actual power curve or pace data, not generic percentages.

Your target CTL

When you set a goal (a race or a training-goal template), the AI calculates the optimal CTL you need to reach by that date, considering your current fitness, time available, and a safe ramp rate (typically 3–7 CTL points per week). The goal progress chart shows your projected trajectory and whether you're on track.

CTL, ATL and TSB

• CTL (Chronic Training Load) — your fitness, built over ~6 weeks.

• ATL (Acute Training Load) — recent fatigue from the past ~1 week.

• TSB (Training Stress Balance) — CTL minus ATL: positive = fresh, negative = fatigued.

Why Form differs from Intervals.icu

Intervals.icu shows the raw TSB value (CTL − ATL). IntervalCoach shows Form as a percentage of your fitness: (TSB ÷ CTL) × 100. For example, with CTL 58 and ATL 88, Intervals.icu shows TSB = −30 while IntervalCoach shows −52%. Both are correct — the percentage shows how fatigued you are relative to your fitness level.

Power and heart-rate zones

IntervalCoach reads your thresholds from Intervals.icu and applies zone bands to them. The bands are different per sport, because a percentage of threshold power and a percentage of threshold speed do not mean the same thing.

Cycling uses the standard Coggan bands against your FTP: Z1 (Recovery) up to 55%, Z2 (Endurance) 56 to 75%, Z3 (Tempo) 76 to 87%, Sweet Spot 88 to 94%, Z4 (Threshold) 95 to 105%, Z5 (VO2max) 106 to 120%, Z6 (Anaerobic) above 121%.

Running uses running-specific bands against your threshold pace (critical speed), which sit much higher: Z1 (Recovery) up to 78%, Z2 (Easy/Aerobic) 78 to 88%, Z3 (Tempo) 88 to 95%, Z4 (Threshold) 95 to 100%, Z5 (VO2max) 100 to 108%, Z6 (Anaerobic) above 108%. Applying the cycling percentages to pace would put your easy runs minutes per kilometre too slow, so we do not.

Swimming uses its own bands against your CSS: Z1 up to 75%, Z2 75 to 85%, Z3 85 to 92%, Z4 (Threshold) 92 to 100%, and above that for faster work.

Custom zone boundaries set in Intervals.icu don't carry over — IntervalCoach uses its own zone definitions.