The short version: one number that predicts injury
Most injuries in amateur sport aren't about a bad step. They're about too much, too soon. You sit still all week, then empty the tank at a weekend half-marathon. Or you bolt two hard interval sessions onto your usual volume. The body can't adapt fast enough — and something gives.
ACWR (acute:chronic workload ratio) is a way to see that imbalance as a number instead of a hunch. In sports science, this is the metric most closely tied to injury risk and overtraining.
Acute and chronic load
The whole idea rests on two time windows:
- Acute load — how much you've trained over the last ~7 days. This is your current week: fresh stress the body is still digesting.
- Chronic load — your average load over ~28 days. This is your accumulated fitness, the base your body is already used to.
ACWR is simply one divided by the other:
ACWR = acute load ÷ chronic load
The logic is deeply physiological. Chronic load shows what your body can handle. Acute load shows what you've just piled onto it. If this week is far above your norm, you've stepped onto ground you aren't prepared for.
What actually counts as "load"
Load isn't kilometres or minutes on their own — it's the internal stress of a session. It's usually expressed through TRIMP (training impulse), which combines duration and heart-rate intensity. An easy hour and an hour of intervals are wildly different loads, even though the clock reads the same. That's why ACWR is built from heart-rate data, not distance.
The safe corridor and the danger zone
The accumulated research gives fairly clear markers:
- 0.8–1.3 — the working corridor ("sweet spot"). Fresh load roughly matches your base. You train enough to progress without outrunning your ability to adapt.
- Below 0.8 — undertraining. You've dropped volume. Sometimes that's deliberate recovery, but week after week fitness fades — and coming back becomes its own spike.
- 1.3–1.5 — the yellow zone. You've added noticeably. Not forbidden, but a reason to watch how you feel and not pile on more.
- Above 1.5 — the red zone. Injury and overtraining risk climbs sharply. You've handed the body a load it wasn't ready for.
The key takeaway is counterintuitive: high chronic load protects you. A well-trained athlete with a solid base survives a hard week far better than a beginner doing the same absolute volume. The danger isn't volume itself — it's a sharp spike relative to your own norm.
Why EWMA, not a plain average
The naive maths — sum the last 7 days, divide by the 28-day average — has a flaw: it assumes a session 27 days ago affects you as strongly as yesterday's. It doesn't. Training stress decays.
So a proper ACWR uses EWMA (exponentially weighted moving average). Recent days carry more weight; older days count exponentially less. This mirrors the physiology of recovery and, in the research, tracks real injury risk better than a flat average.
You can't do this by hand. It needs daily heart-rate data and correct decay maths — automation isn't optional here.
How to use ACWR in practice
- Raise load gradually. A rough guide is around a 10% increase in volume per week. That keeps ACWR inside the corridor.
- Don't confuse a "zero week" with rest. Complete downtime drops your chronic base, so returning to training becomes a spike. Light activity beats zero.
- Plan your peaks on purpose. A race or a hard block will push ACWR up temporarily — fine, as long as recovery follows.
- Watch the trend, not one reading. What matters is direction: are you holding the corridor, or creeping into the red week after week?
Where Sport AI Link fits in
The catch with ACWR is simple: it has to be recalculated every day from heart-rate data, and by hand that's impossible. Sport AI Link does it for you.
The bot connects to your watch (Garmin, Oura, Whoop, Fitbit, Strava), computes TRIMP for every session, and tracks ACWR through EWMA — with deterministic code, no guesswork. The moment the ratio drifts into the yellow or red zone, you get a straight warning before overreaching turns into an injury. The code does the numbers; the AI voices the coaching on top of them, honestly and without sugar-coating.
Connect your device in Telegram and see your ACWR in a couple of minutes — let your body tell you where you're already on the edge.