CTL, ATL and TSB explained — and why your AI coach needs them

If you train with power or heart rate, three acronyms decide most of your decisions: CTL, ATL and TSB. Understanding them is what separates “just riding” from training with intent — and it is exactly the context an AI coach needs to be useful.

CTL — Chronic Training Load (your fitness)

CTL is a 42-day weighted average of your daily training stress. It rises slowly when you train consistently and decays when you rest. Think of it as your fitness: hard to build, slow to lose.

ATL — Acute Training Load (your fatigue)

ATL is the same idea over roughly 7 days. It spikes after hard blocks and drops quickly when you ease off. It is your short-term fatigue.

TSB — Training Stress Balance (your form)

TSB is simply CTL minus ATL. Positive TSB means you are fresh (good for racing); negative TSB means you are loaded (normal during a build). The art is timing: build fitness without digging a hole you cannot climb out of before your goal event.

• Deep negative TSB for too long → overreaching and stale legs.
• Sitting at positive TSB all season → you are not training hard enough.
• A well-timed taper raises TSB into your event so you arrive fit and fresh.

Where AI helps

These numbers are tedious to balance by hand. With ai2workout, your assistant reads live CTL/ATL/TSB from Intervals.icu and proposes the next session that keeps you progressing without burning out — then explains its reasoning before anything hits your calendar.