The three numbers every triathlete should know
If you've spent any time looking at training dashboards, you've probably seen the acronyms CTL, ATL, and TSB. They sound technical, but they're actually straightforward — and incredibly useful for understanding your training state.
CTL: Chronic Training Load (Fitness)
CTL represents your long-term training load — essentially, how fit you are based on the training you've accumulated over the past ~6 weeks (42-day exponentially weighted average).
- High CTL = you've been training consistently and building fitness
- Low CTL = you've been training less or are just starting out
Think of CTL as your "fitness bank account." You build it slowly through consistent training.
ATL: Acute Training Load (Fatigue)
ATL represents your short-term training load — how tired you are based on your most recent ~1 week of training (7-day exponentially weighted average).
- High ATL = you've trained hard recently and are likely fatigued
- Low ATL = you've rested recently
ATL responds quickly. One hard training week will spike your ATL; a rest week will bring it down fast.
TSB: Training Stress Balance (Form)
TSB is the magic number. It's simply CTL minus ATL (fitness minus fatigue).
- Positive TSB = you're rested and fresh (great for race day)
- Negative TSB = you're carrying fatigue (normal during a training block)
- Very negative TSB = you might be overreaching
The sweet spot for race day is typically a TSB between +10 and +25 — fit enough to perform, rested enough to execute.
ACWR: Acute-to-Chronic Workload Ratio
TRI-HARDER also tracks ACWR (ATL ÷ CTL), which tells you how your recent training compares to your long-term baseline:
- Below 0.8 — you're undertrained relative to your baseline
- 0.8 to 1.3 — the optimal training zone
- 1.3 to 1.5 — caution — you're pushing hard relative to fitness
- Above 1.5 — high injury risk — you've ramped up too quickly
How TRI-HARDER uses these metrics
TRI-HARDER computes CTL, ATL, TSB, and ACWR from your actual workout data (via Strava or intervals.icu). These metrics appear on the Training Dashboard and are automatically included in your AI coaching context — so when you ask the coach about your training, it knows your exact fitness and fatigue levels.
They're also included in the PDF coaching report, giving you a snapshot of your training load alongside AI-generated analysis.
Key takeaways
- CTL tells you how fit you are (build it slowly)
- ATL tells you how tired you are (it moves fast)
- TSB tells you if you're fresh or fatigued (target positive for races)
- ACWR tells you if you're ramping up safely (stay in 0.8–1.3)
Understanding these four numbers gives you a powerful lens on your training — and TRI-HARDER puts them front and centre so you don't have to calculate anything yourself.