Training load, explained without the jargon

TSS, CTL, ATL, TSB. Four acronyms that determine whether your next workout makes you faster or breaks you. Here is a plain-English version that fits in five minutes.

TSS — Training Stress Score

A single number per workout. It accounts for both intensity and duration. A 60-minute easy run might be 50; a hard threshold session might be 90; a long ride could top 200. Most platforms compute it from heart-rate zones or power.

CTL — Chronic Training Load (the "fitness")

An exponential rolling average of TSS over ~42 days. CTL goes up when you train consistently and down when you don't. Think of it as how much work your body is acclimatised to.

ATL — Acute Training Load (the "fatigue")

Same idea but on a 7-day window. ATL spikes after hard weeks and falls during taper.

TSB — Training Stress Balance (CTL − ATL)

The difference. Negative TSB means you are accumulating more fatigue than fitness. Positive TSB means you are fresher than your fitness baseline. Race-day target is usually +5 to +25.

How to actually use it

1. Build: CTL trending up, TSB mildly negative.
2. Push week: TSB drops to −15 to −25. Expect to feel rough.
3. Recovery week: Cut volume so ATL drops faster than CTL. TSB rises.
4. Race / test: Target +10 TSB on race morning.

How Radiant Health computes it for you

We pull workouts from Strava, Garmin and Health Connect, compute TSS for each session, and roll it into your CTL / ATL / TSB. The morning briefing tells you which phase of the cycle you are in instead of just dumping numbers.