Training Load Explained: What ATL, CTL and TSB Mean for Your Running

In 1975, exercise physiologist Dr Eric Banister developed a mathematical model to describe how training stress affects athletic performance over time. In the 1990s, Dr Andrew Coggan adapted this into what is now known as the Performance Management Chart — the ATL/CTL/TSB model. It’s used by Olympic coaches, professional cycling teams, and elite marathon programmes worldwide.

The core insight is deceptively simple: fitness = chronic training stress, and fatigue = acute training stress. Your ability to perform at any given moment is the difference between the two. Train consistently, manage fatigue intelligently, and you will get faster. Ignore fatigue and you will overtrain or get injured.

The Three Numbers

CTL — Chronic Training Load (Your Fitness)

CTL is a 42-day exponentially weighted moving average of your daily training stress. It rises slowly with consistent training and falls slowly when you reduce volume. Think of it as your current aerobic fitness base.

A CTL of 30 is a solid recreational runner doing 25–35km per week. A CTL of 60–70 represents a competitive amateur doing 60–80km per week. Elite marathon runners carry CTL values of 120–150+.

ATL — Acute Training Load (Your Current Fatigue)

ATL is a 7-day exponentially weighted moving average of training stress. It responds much faster than CTL — a single hard week can spike your ATL by 20–30 points. ATL represents the accumulated fatigue from recent training.

High ATL is not inherently bad — it’s the signal that you’ve been doing the work. The problem is when ATL rises much faster than CTL.

TSB — Training Stress Balance (Your Form)

TSB = CTL − ATL.

This is the single number most predictive of performance on a given day. A positive TSB means you’re fresh (fitness exceeds fatigue). A negative TSB means you’re carrying more fatigue than your fitness baseline. Both states have their place.

ATL, CTL, and TSB over a 12-week training block with taper

The chart above shows a 12-week training block. Notice how ATL spikes above CTL during hard training weeks (TSB goes negative). During the taper, ATL drops rapidly while CTL barely moves — TSB swings positive, arriving at race day in the fresh, high-performing zone.

The TSB Zones

TSB form zones — where you want to be and why

Most runners should spend the majority of their training year in the Optimal Training Zone (TSB −10 to +10). This is where the best long-term fitness adaptations occur — enough fatigue stimulus to drive adaptation, but not so much that recovery is compromised.

Deeper negative TSB values (below −20) are appropriate during deliberate overreaching phases — short blocks of high-volume training, typically 2–3 weeks maximum, always followed by a recovery week.

TSB above +20 is the taper zone: ideal for race week. CTL has been built; ATL has been allowed to drop. You arrive at the start line fit and fresh.

How to Use Training Load in Practice

Building Your Fitness (CTL)

CTL builds through consistent training over months. The key constraint is the 10% mileage rule — adding too much volume too fast spikes ATL without giving CTL time to catch up, sending TSB deeply negative and raising injury risk. Read more: How to Build a Running Habit Without Getting Injured.

Managing Your Fatigue (ATL)

Monitor ATL weekly. If it’s consistently more than 25–30 points above your CTL, schedule a recovery week. Every 3–4 weeks of progressive training should be followed by one week at 70% of your normal volume.

Peaking for a Race (TSB)

A standard taper for a marathon or half-marathon is 2–3 weeks. In week one of the taper, reduce volume by 20–30%. In race week, reduce to 40–50% of normal volume while keeping some short quality sessions to stay sharp. Watch TSB — aim to arrive race day between +5 and +25.

Common Mistakes

Tapering too hard. Cutting all training in the 2 weeks before a race drops CTL without purpose. Short quality sessions keep CTL stable while letting ATL fall.
Ignoring deeply negative TSB. Running through a TSB of −40 or below is how stress fractures happen. When TSB goes very negative, reduce volume first, then re-evaluate.
Using ATL alone. High ATL is fine if CTL is proportionally high. A CTL of 60 and ATL of 70 (TSB −10) is healthy hard training. A CTL of 30 and ATL of 70 (TSB −40) is a path to injury.

Where Athlr Shows These Numbers

Open the Training tab in Athlr to see your current ATL, CTL, and TSB dashboard. The model uses the same exponentially weighted moving average formula as TrainingPeaks and Strava (which charges $80/year for access to it). Athlr calculates it from your full activity history — free, with no subscription required.

The longer you’ve been tracking with Athlr (or since importing your history from Strava, Garmin, or Apple Health), the more accurate these numbers become. The model needs at least 6 weeks of data to be meaningful, and 12 weeks to be reliable.

See your training load for free

ATL, CTL, and TSB calculated automatically from your activity history. No subscription. No paywall.

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Frequently Asked Questions

What are ATL, CTL, and TSB in simple terms?

CTL (Chronic Training Load) is your current fitness level — built by training consistently over months. ATL (Acute Training Load) is your recent fatigue — how hard you've trained in the past 7 days. TSB (Training Stress Balance) is the difference: CTL minus ATL. Positive TSB means you're fresh; negative TSB means you're carrying fatigue.

What TSB number is ideal for a race?

Most athletes perform best with a TSB between +5 and +25 on race day. This means enough recent training to maintain fitness, but enough recovery to feel fresh. You reach this zone by tapering — reducing training volume 1–3 weeks before your goal race while keeping some intensity.

How quickly does CTL build?

CTL is based on a 42-day exponential moving average, so it builds slowly. You can expect to add roughly 1–3 CTL points per week during consistent training. Going from CTL 30 to CTL 60 (a meaningful fitness jump) takes roughly 3–4 months of consistent work. There are no shortcuts — any attempt to build CTL faster than your body adapts leads to injury.

How do I use training load to avoid overtraining?

Watch your ATL. If it climbs significantly higher than your CTL (TSB going deeply negative — below -30), your fatigue is outrunning your fitness. Reduce volume or take rest days until TSB recovers to -10 or higher. Also watch for pace slowing at the same perceived effort — that's a clinical sign of overtraining before any metric catches it.

Does Athlr calculate training load automatically?

Yes. Athlr calculates ATL, CTL, and TSB automatically from your full activity history — the same model used by platforms like TrainingPeaks, which charges $19/month for it. Athlr's training load dashboard is free, and becomes more accurate the longer your activity history.

What is TSS (Training Stress Score) and do I need it?

TSS is a per-activity metric used in cycling to quantify the training stress of a specific session based on power output. ATL/CTL/TSB are built from accumulated TSS over time. For running without a power meter, Athlr estimates training stress from duration, pace, and heart rate. You don't need to understand TSS to benefit from ATL/CTL/TSB — the charts tell the story.