What Is CTL, ATL, and TSB? The Ultra Runner's Guide to Training Load
Tags: training, performance, CTL, ATL, TSB, training load, PMC, ultra running
---
You've been running for weeks. The miles are stacking. The vert is building. But are you actually getting fitter — or just more tired?
That's the question CTL, ATL, and TSB answer. These three numbers are the engine behind the Performance Management Chart (PMC) in HARDN, and they tell you something no single workout can: where your fitness stands relative to your fatigue.
If you're training for an ultra, these are the most important numbers you're probably not paying attention to.
[CTL_ATL_TSB_CARDS]
---
What Is CTL? (Chronic Training Load)
CTL = Fitness.
CTL is a rolling 42-day exponentially weighted average of your daily Training Stress Score (TSS). It represents the training load your body has adapted to over the past six weeks.
- Higher CTL = more accumulated fitness
- CTL builds slowly — it takes weeks of consistent training to move the needle
- CTL drops fast — miss a week and you'll see it
What's a Good CTL for Ultra Runners?
| CTL Range | What It Means |
|-----------|---------------|
| 20–40 | Recreational runner / early base phase |
| 40–70 | Solid aerobic base for 50K |
| 70–100 | Strong fitness for 50-miler or 100K |
| 100–130 | Peak fitness for 100-miler |
| 130+ | Elite / high-volume athlete |
Your CTL doesn't need to be 130 to finish a hundred. But if your CTL is 45 and your race is in 4 weeks, you have a fitness gap that no taper will fix.
---
What Is ATL? (Acute Training Load)
ATL = Fatigue.
ATL is a rolling 7-day exponentially weighted average of your daily TSS. It represents how much stress your body is carrying right now.
- Higher ATL = more recent fatigue
- ATL spikes quickly after big training weeks
- ATL drops quickly during rest or taper
ATL is the reason you feel wrecked after a peak training week even though your fitness is the highest it's been all season. Your body hasn't recovered from the recent load yet.
Reading ATL
If your ATL is significantly higher than your CTL, you're in a state of acute overreach. That's fine for a week or two during a planned build. It's a problem if it lasts three or four weeks — that's when injuries and burnout happen.
---
What Is TSB? (Training Stress Balance)
TSB = Form. TSB = CTL − ATL.
TSB tells you the balance between your accumulated fitness and your current fatigue. It's the single most useful number for knowing whether you're ready to race.
- Positive TSB → you're rested, fresh, and ready to perform
- TSB near zero → balanced — training hard but recovering
- Negative TSB → you're carrying more fatigue than fitness — not race-ready
TSB Zones for Ultra Runners
[TSB_ZONES]
---
How They Work Together: The PMC
The Performance Management Chart plots CTL, ATL, and TSB over time so you can see your fitness arc across an entire training block.
Here's what a healthy build-to-race pattern looks like:
[TAPER_STEPS]
The magic of the PMC is that it shows you the shape of your preparation. A good taper isn't guesswork — it's watching TSB rise into the green zone while CTL holds steady.
---
How TSS Is Calculated
Every workout generates a Training Stress Score based on:
- Duration — longer workouts = more stress
- Intensity — measured by heart rate relative to your zones
The formula HARDN uses:
> TSS = (Duration in hours) × (Intensity Factor²) × 100
Where Intensity Factor is derived from your average heart rate relative to your lactate threshold heart rate (LTHR). A 60-minute easy run might produce a TSS of 40-50. A 4-hour mountain long run could generate 250+.
---
Common Mistakes Ultra Runners Make with PMC Data
1. Ignoring TSB Before a Race
You trained hard. CTL is high. But your TSB is −15 on race morning. You're going to suffer more than necessary. A proper taper should bring TSB to +15 to +25 by race day.
2. Panicking When CTL Drops During Taper
CTL will drop 5-10% during a 2-week taper. That's fine. You're not losing fitness — you're shedding fatigue. The TSB gain more than compensates.
3. Chasing CTL Year-Round
High CTL all the time leads to staleness and injury. CTL should peak before your A-race and drop during recovery blocks. Periodization matters.
4. Not Wearing a Heart Rate Monitor
Without HR data, TSS estimates are less accurate. Strap or optical — just wear one on every run. Your PMC data will be dramatically more useful.
---
How HARDN Uses These Numbers
Your HARDN Score™ incorporates CTL, ATL, and TSB as core inputs to your race readiness rating. The Performance Management Chart on your Training page shows all three lines plotted over time, with your current values, trend direction, and a status indicator (Fresh / Balanced / Fatigued / Overreached).
When you generate an AI training plan, HARDN uses your current CTL as the starting fitness baseline and plans weekly TSS targets to build CTL progressively toward your goal race. The adaptive recalibration system compares planned vs. actual TSS each week and adjusts forward if you're ahead or behind.
---
The Bottom Line
- CTL tells you how fit you are.
- ATL tells you how tired you are.
- TSB tells you if you're ready to race.
Three numbers. One chart. The clearest picture of your training you'll ever get.
Open your [Performance Management Chart](/training) in HARDN and start paying attention. Your next PR is hiding in those lines.