What Is CTL? Training Load Explained: CTL, ATL, TSB

Greg · UpGreg AI Coach Published June 8, 2026 5 min read

Your watch shows you acronyms, your coaching app adds more, and nobody ever explained what they concretely mean. Here are the only three numbers you need to understand how your body absorbs training — explained like a bank account.

First: every workout has a cost, the TSS

Every session generates a stress score (TSS — Training Stress Score) combining its duration and intensity. One easy hour ≈ 50 points. One very hard hour ≈ 100. It's the base currency for everything else.

CTL — your fitness (the savings account balance)

CTL (Chronic Training Load) answers one question: what is your current fitness, based on your real training? It's the weighted average of your load over the last 42 days — your endurance capital: everything you've deposited, session after session, over the past month and a half. It rises slowly — impossible to cheat — and falls slowly too: one week off doesn't ruin six months of work.

ATL — your fatigue (this month's spending)

ATL (Acute Training Load) answers the other question that matters: how fresh or tired are you right now? It's the same average as CTL, but calculated over 7 days only — what you've just spent. It moves fast — a big week sends it soaring, three quiet days bring it back down.

TSB — your freshness (what's left in the checking account)

TSB (Training Stress Balance) answers the question that decides your day: fresh or depleted? The formula is simple: CTL − ATL, fitness minus fatigue. This is the number to watch:

Very negative TSB (−20 and beyond): you're digging. Normal in a loading block, dangerous if it lasts.
Slightly negative TSB (−15 to 0): the productive zone — training enough to progress, not enough to bury yourself.
Positive TSB (+5 to +15): you're fresh. That's where we want you on race morning.
Very positive TSB for long: you're rested… and detraining.

And ACWR, the anti-injury guardrail

One last useful acronym: ACWR (acute:chronic workload ratio) compares your current week to your recent average. Beyond ~1.3 — a week 30% bigger than usual — injury risk shoots up. It's THE number coaches watch to slow down Monday enthusiasts.

Frequently asked questions

What is CTL, in one sentence?

CTL — Chronic Training Load — is your current fitness expressed as a single number: the weighted average of your training stress over the last 42 days. The CTL meaning boils down to this: the higher it climbs safely, the more training your body can absorb without breaking down.

What does CTL mean in training?

CTL stands for Chronic Training Load: a weighted average of your training stress over the last 42 days. It's the standard way to measure current fitness, popularized by tools like TrainingPeaks — and it's exactly what UpGreg calculates automatically from your Strava or Garmin activities.

Is a higher CTL always better?

No. CTL only shows the volume and intensity you've accumulated over time, not your readiness to perform right now. A CTL built up too fast, without matching recovery, raises injury risk (see ACWR above). What matters is the balance between CTL and ATL — that's exactly what TSB measures.

What's the difference between CTL, ATL and TSB?

CTL is your fitness (42-day average load), ATL is your fatigue (7-day average load), and TSB is the gap between the two (CTL − ATL) — your freshness. Greg tracks all three after every imported activity and adjusts your plan before the gap gets dangerous.

🗣️ Greg's take

“You don't need to compute any of this — that's my job. I watch your CTL, ATL, TSB and ACWR on every imported activity, and I adjust the plan before you dig too deep. You only need to remember one thing: if I give you a quiet week while you feel like you've grown wings, it's because your bank account says something your ego doesn't. Ego doesn't pay the physio's bills.”

Connect Strava and see your fitness, fatigue and freshness computed automatically.

No credit card · Strava connected in 30 s

Understanding AI sports coaching →