Your body has a budget. You can spend fitness — run hard, add miles, push intervals. And you accrue debt — fatigue, wear, the need to recover. Training load science gives us numbers for both sides of that ledger.
ATL — Acute Training Load is your 7-day rolling average of training stress. Think of it as how tired you are right now. High ATL means you've been working hard recently.
CTL — Chronic Training Load is your 42-day rolling average. This is your fitness. It builds slowly, and it's what makes you faster and stronger over months of consistent training.
TSB — Training Stress Balance is the difference: CTL minus ATL. When TSB is positive, you're fresh — more fitness than fatigue. When it's negative, you're in a training block, building fitness at the cost of feeling tired.
| TSB | What it means | |-----|---------------| | > +15 | Fresh — ready for peak effort | | 0 to +15 | Race-ready | | -10 to 0 | Optimal training stress — this is where progress happens | | -20 to -10 | Fatigued — be careful | | < -20 | Overtrained — stop, rest, reassess |
The goal of a training plan is to spend most of your time in the -10 to 0 zone, with periodic recovery weeks that bring TSB back toward positive. Then peak for your race at +5 to +15.
Most runners never see these numbers. Their coach calculates them in the background and adjusts the plan accordingly — which is exactly what Miles does.
When you connect Strava, Miles calculates your ATL, CTL, and TSB after every run. If you're drifting toward -25, he'll add a recovery day before you ask for one. If your fitness has been building nicely for six weeks, he might suggest it's time to test yourself with a hard tempo.
Here's what the numbers can't capture: you also had three hours sleep because the baby was up, and your work presentation is tomorrow.
That's why Miles asks. The numbers are the science. Your context is the coaching. Together, they make a plan that's actually yours.