If you've spent time on running forums or training apps, you've probably seen acronyms like CTL, ATL, and TSB thrown around. They sound technical, but the concepts behind them are simple — and incredibly useful for training smarter.
Let's break them down in plain English.
The big idea: stress and recovery
Every workout creates stress on your body. Easy runs create a little. Hard intervals create a lot. Your body adapts to this stress over time — that's how you get faster.
The key insight is that fitness and fatigue both accumulate, but at different rates:
- Fitness builds slowly and lasts a long time (weeks to months)
- Fatigue builds quickly but dissipates fast (days)
Training load metrics quantify this relationship so you can make better decisions about when to push and when to rest.
TSS: Training Stress Score
Before we get to the big three, you need to understand TSS — Training Stress Score. This is a single number that represents how much stress a workout created.
TSS accounts for both duration and intensity. A 30-minute tempo run might score similarly to a 60-minute easy run, because the higher intensity compensates for the shorter duration.
TSS can be calculated from:
- Heart rate (hrTSS): Uses your average heart rate relative to your lactate threshold heart rate
- Power (if you run with a power meter): Uses functional threshold power
- Pace (rTSS): Uses your pace relative to threshold pace
Most platforms, including Pairform, use heart rate-based TSS since it works with any chest strap or wrist-based HR monitor.
CTL: Chronic Training Load (your "fitness")
CTL is your average daily TSS over the last 42 days. Think of it as your fitness bank account — the bigger it is, the more training your body can handle.
- A CTL of 30 means you've been averaging about 30 TSS per day over 6 weeks
- A higher CTL means more fitness
- CTL changes slowly — you can't crash-train your way to a high CTL in a week
What to watch for:
- CTL should ramp gradually. A common guideline is no more than 5-7 points per week
- If your CTL drops significantly, you're losing fitness (which is fine during planned recovery)
ATL: Acute Training Load (your "fatigue")
ATL is your average daily TSS over the last 7 days. It represents how tired you are right now from recent training.
- ATL spikes after hard training weeks
- ATL drops quickly during rest days and recovery weeks
- A high ATL relative to your CTL means you're carrying a lot of fatigue
TSB: Training Stress Balance (your "form")
Here's where it gets powerful. TSB is simply:
TSB = CTL - ATL
In other words: form = fitness minus fatigue.
- TSB is positive → You're rested. Your fitness exceeds your recent fatigue. Good for racing.
- TSB is negative → You're fatigued. You're building fitness but carrying accumulated stress.
- TSB around zero → Balanced. Neither particularly fresh nor fatigued.
TSB ranges and what they mean
| TSB Range | What it means | What to do |
|---|---|---|
| +15 to +25 | Peak form | Race day territory |
| +5 to +15 | Fresh | Good for hard sessions |
| -10 to +5 | Balanced | Normal training |
| -10 to -30 | Fatigued | Expected during build weeks |
| Below -30 | Overreaching | Back off or risk overtraining |
How to use training load in practice
Planning a taper
The classic race taper is all about TSB management. You want to maintain CTL (fitness) while dropping ATL (fatigue), which pushes TSB positive.
A typical 2-week taper:
- Week 1: Reduce volume by 30-40%, keep some intensity
- Week 2: Reduce volume by 50-60%, a couple of short sharpening workouts
- Race day: TSB between +15 and +25
Avoiding overtraining
If your TSB stays deeply negative for more than 2-3 weeks, you're at risk for overtraining. Watch for:
- Declining performance despite maintained training
- Poor sleep quality
- Elevated resting heart rate
- Persistent fatigue and mood changes
Building a training block
A good 4-week training block might look like:
- Weeks 1-3: Progressive overload (TSB trends negative as ATL rises)
- Week 4: Recovery week (TSB rebounds toward zero or positive)
Where to track training load
To calculate training load, you need:
- Consistent workout data with heart rate (Strava, Garmin, etc.)
- Your lactate threshold heart rate (LTHR) for accurate TSS
- A platform that computes the math automatically
Pairform auto-calculates TSS, CTL, ATL, and TSB from your Strava data, so you can just check your dashboard and see where you stand. No spreadsheets required.
Common mistakes
Chasing CTL: Don't try to maximize CTL. A CTL of 60 with good consistency beats a CTL of 80 built on junk miles and injury risk.
Ignoring qualitative signals: TSB says you're fresh, but your legs feel dead? Trust your body. The numbers are a guide, not gospel.
Not setting your LTHR: TSS calculations are only as good as your threshold input. Get a proper threshold test or use a recent hard race effort to estimate.
The bottom line
Training load metrics give you a framework for the decisions runners have always had to make: when to push, when to rest, and when to race. They don't replace listening to your body, but they add a powerful data layer to your training.
Start tracking, learn your patterns, and you'll make better training decisions — consistently.
Want to see your training load numbers? Connect your Strava to Pairform and we'll calculate everything automatically.