Training Science Glossary

Aithlo speaks a little training-science language. These are the core terms you'll see across your dashboard, plans and coach chat — explained in plain English.

Load and balance metrics#

These four numbers work together to describe how hard you're training and how fresh you are.

TSS — Training Stress Score#

A single number that captures how hard one session was, combining intensity and duration. A short easy jog scores low; a long hard ride scores high. Aithlo calculates TSS from the best data available, in priority order: power, then heart rate, then pace, then swim speed, with a fallback if none are present. Every activity gets a TSS, and those scores feed everything below.

CTL — Fitness#

Short for Chronic Training Load, CTL is a 42-day rolling average of your TSS. Because it averages over six weeks, it rises slowly and represents the durable fitness you've built. Think of it as the big, slow-moving engine.

ATL — Fatigue#

Short for Acute Training Load, ATL is a 7-day rolling average of your TSS. It reacts quickly to recent training, so a hard week pushes it up fast and a rest week brings it down just as quickly. This is your fatigue.

TSB — Form#

Short for Training Stress Balance, TSB is simply Fitness minus Fatigue (CTL − ATL). A positive number means you're rested and fresh; a negative number means you're carrying fatigue. This is your form — how ready your body feels to perform.

Readiness#

Readiness translates your form (TSB) into a plain-language status that guides and adapts your training. The bands are:

• Fresh — TSB above 25. Very rested; great for racing, or a sign you may be undertraining.
• Optimal — TSB between 5 and 25. The sweet spot for performing well.
• Neutral — TSB between −10 and 5. Balanced; normal training territory.
• Building — TSB between −25 and −10. Productively loaded; you're absorbing training.
• Fatigued — TSB below −25. Carrying heavy fatigue; ease off and recover.

See training load and trends to watch these move over time.

Periodization phases#

Periodization is the shape of a training block over time, automatically aligned to your races:

• Base — build aerobic foundation and consistency.
• Build — add intensity and event-specific work.
• Peak — sharpen toward top fitness.
• Taper — reduce load so you arrive fresh on race day.
• Recovery — lighter blocks that let adaptations settle.

Learn more in training phases.

A / B / C race priority#

Not every race matters equally. You label each race by priority so the plan knows where to aim:

• A race — a key goal the plan peaks and tapers for.
• B race — important but secondary; you train through it with light freshening.
• C race — a tune-up or for fun, done on tired legs without disrupting the plan.

With the vocabulary down, explore your dashboard overview to see these numbers in action.