Training load and ACWR, explained for runners
The acute to chronic workload ratio, or ACWR, compares how much you trained in the last week with how much you have been used to over the last four. It is a simple way to spot a spike in training before it becomes an injury or a bad week.
The formula
Acute load is your training load over the last 7 days. Chronic load is your average weekly load over the last 28 days. The ratio is acute divided by chronic. A ratio of 1.0 means this week looks like your recent normal. Above 1.0 means you did more than usual. Below 1.0 means you did less.
How to read the number
- About 0.8 to 1.3. Steady building. This range is commonly cited as the comfortable zone.
- Above about 1.3. A fast ramp. Worth watching, and a good time to keep easy days truly easy.
- Well above 1.5. A large spike relative to what your body is used to.
- Below about 0.8. You are doing less than usual. That is expected in a taper or after a break.
Treat the ratio as a flag, not a prediction. Research on ACWR is debated, and no single number tells you whether you will get hurt. It is most useful for catching a jump you did not notice yourself.
The new-runner problem
The simple formula breaks for anyone with less than four weeks of history. If the chronic number is divided by four weeks that do not exist yet, it comes out far too small and the ratio explodes. A runner doing the same four 40-minute sessions every single week would read about 4.0 on day seven, 2.0 on day fourteen, and only settle at 1.0 on day twenty-eight. Nothing about their training changed. Only the calendar did.
YUHA divides by the weeks of data you actually have, and it shows no ratio at all until it has seen at least a week. That keeps a new account, or a runner returning from a break, from being told to rest because of a signup date.
What to do with a high ratio
A high ratio is a reason to adjust, not to stop. Keep the next quality session shorter or swap it for steady running, protect sleep, and let the load settle back toward your normal. YUHA feeds the ratio into your readiness score and into how your training plan eases the weeks ahead.
Common questions
What is a good ACWR for runners?
A ratio of roughly 0.8 to 1.3 is commonly cited as the comfortable range for steady building. Above about 1.3 is a fast ramp worth watching. Treat it as a flag rather than a guarantee.
Why is my ACWR so high when I just started?
With under four weeks of history, the simple formula divides by weeks that do not exist yet and overstates the ratio. YUHA divides by the weeks it has actually seen and hides the ratio until it has at least a week of data.
Does ACWR predict injury?
No. Research on the ratio is mixed. It is a simple way to notice a sudden jump in training, not a forecast of injury.
Which devices does YUHA take training load from?
Garmin's own load figures when available, otherwise a duration-based load built from your activities across Strava, Apple Health, Health Connect and manual logs.