Definition
TSS (Training Stress Score) quantifies the physiological cost of a workout using intensity and duration. CTL (Chronic Training Load) is a rolling measure of fitness—your 6-week accumulated stress. ATL (Acute Training Load) tracks recent fatigue—typically a 7-day weighted average of TSS. The difference between CTL and ATL gives you form (TSB): how fresh or tired you are.
Why It Matters
These three numbers turn subjective effort into objective signals. Use them to judge whether you’re getting fitter, predict race-day freshness, and avoid overtraining or chronic underload. For club riders chasing sustainable gains, they help schedule hard days, recovery weeks, and tapering with measurable outcomes.
Short lead-in line.
Powermeters — measure the power data that produces accurate TSS
Power meters & computers — pair for in-ride metrics and auto-TSS calculation
Trainers & rollers — structured indoor sessions that reliably create target TSS
Training tools — software and apps to track CTL/ATL over weeks
Heart rate monitors — complementary load data when power isn’t available
How It Works
TSS is calculated from intensity factor (IF) and duration. In practice, IF = workout normalized power ÷ FTP. TSS = (seconds × normalized power × IF) ÷ (FTP × 3600) × 100, simplified: higher power or longer time raises TSS. That makes a 2-hour tempo ride and a short threshold set produce very different stress profiles.
CTL and ATL are exponential weighted averages of daily TSS. CTL uses a long time constant (commonly 42 days) to reflect fitness accumulation. ATL uses a short time constant (commonly 7 days) to reflect immediate fatigue. The simple mental model: TSS = one workout’s bookkeeping; CTL = your fitness bank balance; ATL = how overdrawn that account is today.
Diagram (visual idea): picture two curves—one slow-rising (CTL) and one spiky and fast (ATL). Hard sessions push ATL up; consistent work nudges CTL higher. When ATL crosses above CTL, you feel fatigued; when CTL outruns ATL, you feel fresh and race-ready.
Pro tip:
Use a power meter and training app together. Automated TSS from a power computer removes calculation guesswork and keeps CTL/ATL consistent.
Key Specs/Thresholds
Typical time constants: CTL uses 42 days; ATL uses 7 days (standard TrainingPeaks convention). Values reported in the app assume those constants.
TSS units: arbitrary score; 100 TSS ~= one hour at FTP for most riders. Example: 2 hours at FTP ≈ 200 TSS.
CTL magnitudes: recreational riders might sit at 20–60 CTL; strong club racers often sit 60–100+ depending on discipline and season.
Acute-to-Chronic Ratio (ACR): ATL / CTL. Values >1.2–1.3 indicate rising fatigue and increased injury/illness risk if sustained. Values <0.8 suggest undertraining.
TSB (form) = CTL − ATL: positive values mean freshness, negative values mean fatigue. Many target a small negative TSB during a training block and a modest positive TSB for a short taper.
Common Misconceptions
TSS is not a perfect measure of “quality.” It quantifies stress, not skill, neuromuscular strain, or race tactic load (e.g., crit surges).
CTL isn’t a fixed “fitness score” across riders. A CTL of 70 for one rider isn’t the same physiological level for another—body size, FTP, and training history matter.
Heart-rate-only TSS substitutes exist, but they underestimate short, high-intensity intervals and overestimate long aerobic steady rides.
Bigger TSS every day isn’t better. Consistent, progressive load with recovery creates lasting CTL gains.
When to Choose/Use
If you use a power meter: rely on true TSS and CTL/ATL as primary training signals.
If you don’t have power: use heart-rate-based TSS cautiously and accept less precision; consider perceived exertion logging alongside.
If planning a peak: raise CTL progressively over months, use short training blocks that increase ATL temporarily, then drop ATL in a taper to yield positive TSB at race day.
If injury or illness appears: drop weekly TSS by 30–50% and consult a coach/physio before resuming ramp-ups.
Sources
TrainingPeaks (2020) — explains CTL/ATL/TSB conventions and time constants.
TrainingPeaks Knowledge (2018) — TSS formula and practical examples.
Allen & Coggan, Training and Racing with a Power Meter (2019) — foundational principles on power-based training.
Key Takeaways
TSS = workout stress; CTL = long-term fitness; ATL = short-term fatigue.
Power meters provide the most accurate TSS; HR-based TSS is a fallback.
Watch ACR (ATL/CTL) to avoid ramping too hard and risking injury.
Use CTL/ATL with a plan: build CTL progressively, then taper to peak.
FAQs
Do I need a power meter to use TSS/CTL/ATL?
No. Power is the most accurate input for TSS and thus CTL/ATL, but heart-rate-based TSS and perceived exertion can be used. Expect less precision without power, especially for short high-intensity efforts.
How fast should I increase CTL without risking injury?
Increase CTL conservatively—rough guidance is 5–10% per week in a multi-week block. Watch the acute-to-chronic ratio (ATL/CTL); values above ~1.2–1.3 for extended periods raise injury and illness risk.
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