TSS, CTL, and ATL: What They Mean and How to Use Them in Your Training
If you've ever opened TrainingPeaks or looked into advanced cycling and triathlon metrics, you've seen these acronyms: TSS, CTL, ATL, TSB. They look like airport codes, but behind them lies a mathematical model that summarizes how your body responds to training load.
In this article I'll explain what each acronym means, how they're calculated, what practical information they give you, and how to use them to make better training decisions.
TSS: Training Stress Score
TSS is a measure of the stress a training session places on your body. It was developed by Dr. Andrew Coggan and combines two factors: duration and intensity relative to threshold.
The conceptual formula is:
TSS = (duration in seconds × Normalized Power × Intensity Factor) / (FTP × 3600) × 100
In practical terms:
- 1 hour exactly at your FTP = 100 TSS points.
- 1 hour in zone 2 (60–65% FTP) = ~40–50 TSS.
- 1 hour with hard intervals at 110–120% FTP = ~80–90 TSS.
- 3 hours of long easy riding in zone 2 = ~150–180 TSS.
TSS doesn't simply measure distance or time: it measures the actual impact on the body based on intensity.
CTL: Chronic Training Load
CTL is the weighted average of your TSS over the past 42 days (approximately 6 weeks). It represents your aerobic fitness level: how much training your body can sustainably tolerate.
A high CTL means you've been training consistently for weeks and your body is adapted to a high load. A low CTL means you've come from a period of little training or recovery.
CTL is calculated with an exponentially weighted moving average with a time constant of 42 days. In simple terms: training from 42 days ago carries very little weight; yesterday's training carries a lot.
ATL: Acute Training Load
ATL is the weighted average of your TSS over the past 7 days. It represents recent accumulated fatigue: how much you've trained in the last week.
If CTL is your "long-term fitness," ATL is your "short-term fatigue." When ATL is significantly higher than CTL, you're accumulating more fatigue than your body is adapted to handle.
TSB: Training Stress Balance
TSB is the difference between CTL and ATL:
TSB = CTL − ATL
TSB represents "form" in a broad sense: if it's positive, you're rested but potentially losing some fitness; if it's negative, you're fatigued but adapting.
| TSB | State | Best for |
|---|---|---|
| +15 to +25 | Fresh, well recovered | Racing, performance tests |
| 0 to +15 | Optimal form with slight fatigue | Quality workouts, key competitions |
| -10 to 0 | Moderate fatigue, adapting | Normal training load phase |
| -20 to -30 | High fatigue | Targeted load weeks, use caution |
| Below -30 | Overreaching | Signal to urgently reduce load |
How to Use These Metrics in Your Planning
To build CTL sustainably
CTL can only rise sustainably by 3–5 points per week in well-trained athletes, and 5–8 points in beginners. Rising faster means ATL spikes and the risk of overtraining or injury increases.
To plan tapering before a race
In the 2–3 weeks before an important competition, reduce volume so ATL drops and TSB rises toward the +10 to +20 range on race day. This process is the taper: you arrive with the training adaptations (high CTL) and the fatigue discharged (low ATL).
To identify weeks of excessive load
If your TSB has been below -25 for more than 10 consecutive days, you probably need a recovery week. Don't wait for physical symptoms: the data is already telling you.
Limitations of the TSS/CTL/ATL Model
- It only measures cardiovascular and power-based load. It doesn't capture muscular stress, sleep quality, or external life stress.
- It requires an up-to-date FTP. If your FTP is outdated, all calculations are incorrect.
- It doesn't distinguish between types of fatigue. 100 TSS points from sprint intervals generates a different fatigue than 100 TSS points from a long zone 2 ride.
These metrics are a powerful tool, not an absolute truth. Used in combination with how you feel, your resting HR, and your actual performance in sessions, they paint a complete picture of your state.
Iron Buddy analyzes your Strava data and evaluates the load trends from your recent weeks. While it doesn't use the exact TSS model (which requires a power meter), it applies an equivalent analysis based on HR and duration to give you concrete information on whether your training load is sustainable and consistent with your goal.
Connect your Strava to Iron Buddy and analyze the load trend of your training.