Training Load: The Number That Predicts Injuries
You can't feel overtraining until it's too late. But there's a metric that sees it coming weeks ahead — and most runners completely ignore it.
Nobody plans to get injured. It just... happens. One day your knee feels off. Or your achilles tightens up. Or you wake up so fatigued you can't face the thought of running. "Bad luck," you tell yourself.
It’s almost never bad luck. In most cases, the data saw it coming. You just weren’t looking at the right number.
What is training load?
Every workout produces a number called training load — a single score that captures how hard the session was on your body. It combines duration and intensity into one value. A 30-minute easy jog might score 25. An hour of intervals might score 90.
Different platforms calculate it differently — Garmin calls it “Training Effect,” intervals.icu uses “Training Load (TSS)” — but the concept is the same: one number per session that tells you how much stress you just applied.
You can't compare a 10km run to a 45-minute bike ride using distance or time alone. Training load normalises everything into the same currency. Now you can compare a run, a ride, and a strength session on equal terms.
The weekly load picture
A single session’s load doesn’t tell you much. What matters is the weekly total — and how it compares to recent weeks.
This is where injuries hide. Not in a single hard session, but in the pattern of load over time. Research consistently shows that the biggest risk factor for overuse injuries isn’t how hard you train — it’s how quickly you ramp up.
The widely cited guideline: don’t increase your weekly load by more than 10-15% week over week. Jump from 200 to 350 in one week? That’s a 75% spike. Your tendons, bones, and connective tissue can’t adapt that fast — even if your cardiovascular system handles it fine.
Cardio fitness adapts in days. Tendons and bones adapt in weeks to months. You feel ready for more volume before your body is ready. That gap is where injuries live.
Acute vs. chronic load
Sports scientists break this down further with two concepts:
Acute load is your training in the last ~7 days. Think of it as recent stress — what you’ve done this week.
Chronic load is your average over the last ~28 days. Think of it as what your body is used to — your training baseline.
The ratio between them — acute ÷ chronic — is the danger zone indicator.
| Ratio | What it means | Risk level |
|---|---|---|
| 0.8 – 1.3 | Load matches your baseline | Low — sweet spot |
| 1.3 – 1.5 | Doing more than usual | Moderate — monitor closely |
| > 1.5 | Significant overload | High — injury risk spikes |
| < 0.8 | Doing less than usual | Detraining — fitness drops |
This is basically what I'm doing when I check your Fitness and Fatigue numbers. Fitness is your chronic load. Fatigue is your acute load. When Fatigue spikes way above Fitness, I start suggesting easy days. The ratio tells me when you're pushing your luck.
The return-from-rest trap
Ironically, one of the riskiest moments is coming back from time off. Two weeks of holiday, a week of illness, a deload week you extended because life got busy.
Your chronic load drops. Your baseline resets lower. Then you come back and try to pick up where you left off. Suddenly you’re running at a load your body was used to but isn’t anymore.
You took 10 days off. Your Fitness (chronic load) dropped from 62 to 48. If you jump back in at your old volume, that's a massive acute:chronic spike. Start at 60-70% of your pre-break load and build back over 2-3 weeks.
After any break longer than a week, cut your first week's volume to 60% of your pre-break average. Then build back 10-15% per week. It feels slow. It prevents the injury that would cost you another month.
What Flott shows you
The weekly load chart on Flott’s dashboard shows this week vs. last week, broken down by sport. It’s deliberately simple — you can see at a glance whether you’re building, holding, or overdoing it.
Flotti uses the same data to flag when your load is spiking too fast. Not after the injury. Before it.
Most injuries aren't random — they follow a predictable pattern of too much, too fast. Training load tracks the stress you're applying. The acute-to-chronic ratio tells you when you're in the danger zone. Build gradually, respect the 10-15% rule, and be especially careful after time off.
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