Training load explained: CTL, ATL, and TSB for cyclists

Training load metrics help you balance fitness, fatigue, and form without guessing. If you have a power meter or structured workouts, you’ve seen acronyms like CTL, ATL, TSB, and TSS. Here’s how they work, what they mean on the bike, and how to use them to plan builds, recovery weeks, and tapers.

The building blocks: TSS, CTL, ATL, and TSB

Most platforms use a power-based score called training stress score (TSS) to quantify a ride. If your functional threshold power (FTP) is set correctly, TSS scales with duration and intensity.

TSS (per ride) ≈ hours × IF² × 100
where IF = intensity factor = NP ÷ FTP

From daily TSS, three rolling metrics are calculated using exponentially weighted moving averages (EWMAs):

• CTL (chronic training load): ~42-day EWMA of TSS. A proxy for aerobic fitness and durability built over weeks.
• ATL (acute training load): ~7-day EWMA of TSS. A proxy for recent fatigue.
• TSB (training stress balance): CTL minus ATL. A proxy for freshness or “form.” Positive numbers mean fresher; negative numbers mean tired.

Names vary by software (e.g., fitness/fatigue/form, BikeScore-based metrics), but the idea comes from the Banister impulse–response model: fitness rises with training, fatigue rises faster and fades quicker.

Important: These are models, not lab values. They don’t know your sleep, life stress, strength training, or illness. Combine them with how you feel and how you perform.

How to use CTL, ATL, and TSB week to week

• Set FTP correctly. Recheck every 4–8 weeks or when workouts feel off. If FTP is too high, TSS is undercounted and CTL looks lower than it should.
• Grow CTL with safe ramp rates. As a rule of thumb:
  • Newer riders: +3 to +5 CTL per week
  • Experienced riders: +5 to +8 CTL per week
  • Cap aggressive blocks around +8 to +10 only for short periods
• Plan recovery weeks. Every 3–4 weeks, reduce weekly TSS by 30–50%. This lets ATL fall, TSB rise toward -10 to +10, and fitness consolidate.
• Taper with a TSB target, not a guess. Reduce TSS 40–60% in the 7–10 days before a key event. Typical day-of ranges:
  • Criterium/road race: TSB around -5 to +10
  • Gran fondo/TT: TSB around 0 to +15 (some riders like +10 to +25 for very long events)
  • Stage race: start less positive (e.g., -10 to 0) to keep “race hardness”

  Individual responses vary—track what worked.

• Match intensity to your goals. CTL rises from any TSS, but fitness quality depends on distribution:
  • Endurance (zone 2): backbone volume
  • Tempo/sweet spot: efficient CTL gains with moderate strain
  • VO2max/anaerobic: peak performance, higher ATL cost
• Account for non-bike stress. Strength training, hard group rides, and life load can push true fatigue beyond ATL. If in doubt, lower your weekly ramp or add an extra rest day.
• Watch red flags. Living below TSB -20 for many days often precedes stagnation or illness. If mood, sleep, or HRV worsen, back off regardless of what CTL says.

Common pitfalls and how to fix them

• Chasing CTL for its own sake. A higher CTL is not automatically faster. Judge progress by power–duration PRs, workout execution, and how you feel at target race intensities.
• Wrong FTP. If endurance rides average IF 0.75+ or threshold workouts demand IF >1.05, your FTP may be low. If threshold work feels impossible at IF ≈0.95–1.00, FTP may be high. Adjust and re-estimate TSS.
• Ignoring recovery. CTL plateaus or falls when you skip rest. Build weeks create fatigue; recovery weeks convert it into fitness.
• Only using power. Pair metrics with session RPE, heart rate decoupling, and simple morning checks (sleep quality, resting HR, HRV if you track it).
• Underestimating single big rides. A huge Saturday can spike ATL and drive TSB way down. Plan an easy day after and a lighter week if needed.
• Not counting cross-training. If you lift or run, consider adding estimated TSS (or use rTSS/HR-based scores) so ATL better reflects total load.

Example: an 8-week build using CTL, ATL, and TSB

Assume you start at CTL 45 and average 6–8 hours per week.

• Weeks 1–3 (build): Aim +4 to +6 CTL per week with two key sessions (one tempo/sweet spot, one VO2) plus endurance. Expect TSB to drift to -10 to -25 by the end of week 3.
• Week 4 (deload): Cut TSS ~40%. ATL drops, TSB rises into -10 to +5. Keep some intensity touches, reduce volume.
• Weeks 5–7 (build): Another +5 to +7 CTL per week with race-specific work. Monitor mood, legs, and sleep; if TSB stays <-25 and legs go flat, pull one session.
• Week 8 (taper/event): Reduce TSS 40–60% over 7–10 days. Keep short, sharp efforts. Target day-of TSB near 0 to +10.

Quick answers

• What CTL should I aim for? The “right” CTL is the one you can sustain while hitting key workouts and feeling good. Many competitive amateurs race well between 50 and 90.
• Is negative TSB bad? Short stretches of -10 to -30 during a build are normal. If it stays very negative and performance or wellbeing drops, unload.
• Can I use HR-based scores? Yes. TRIMP or rTSS can track load when power isn’t available. Consistency matters more than the exact metric.
• Indoor vs outdoor? Calibrate devices and be consistent. Normalized power and IF can differ indoors; compare like with like.

Use CTL, ATL, and TSB as a dashboard: guide your weekly ramp, schedule recovery, and time your taper. Combine the data with how your legs feel and what the power file says, and you’ll turn numbers into better rides.