Concepts
ACWR
The acute:chronic workload ratio: what it compares, the four bands, how it behaves before you have 28 days of history, and what it does not measure.
The acute:chronic workload ratio (ACWR) compares your recent training with the baseline you have built. It divides acute load by chronic load, both of which come from training load. It is the load lens on the Overview chart.
What it compares
| Value | What it is | Reference window |
|---|---|---|
| Acute load | Recent training, weighted towards the last few days | 7 days |
| Chronic load | The baseline built over weeks, what you are adapted to | 28 days |
| ACWR | Acute divided by chronic |
A ratio of 1.0 means recent training matches the baseline. Above 1.0 the week is heavier than the baseline; below it, lighter.
Both values are exponentially weighted rather than flat averages, so a session's contribution fades day by day instead of dropping out of the window all at once Williams et al., 2017. A rest day counts as a rest day, not as a gap in the data.
The four bands
| Ratio | Label | What it describes |
|---|---|---|
| Below 0.8 | Low Load | Recent training is below the baseline. Sustained, this is detraining. |
| 0.8 to 1.3 | Optimal | Recent training is in line with what you are adapted to. |
| 1.3 to 1.5 | Caution | Load is climbing faster than the baseline supports. |
| Above 1.5 | High Risk | A sharp increase relative to the baseline. |
Only Caution and High Risk are colored. Low Load and Optimal are drawn neutral, so a chart with no color means no band needs attention.
The bands come from research in team sports, where a high chronic load was associated with lower injury risk and a rapid increase on a thin base with higher risk (Hulin et al., 2016; Gabbett, 2016).
Projected ACWR
Planned workouts carry a projected load, so the ratio extends past today. The projected part of the line is drawn dashed. It shows where the current plan takes the ratio before you run it, which is what makes a ramp adjustable while there is still time to adjust it.
Completing a workout replaces its projected load with the measured value, and the ratio for that day is recalculated.
Before you have 28 days of history
Chronic load needs 28 days to mean anything. Until then there is no baseline to divide by, so Runtool sizes both windows to the training that exists and marks the ratio as still building. A ratio in the building phase describes a short history and moves more than an established one.
The same applies after a break. A gap of about three weeks with no training decays the chronic baseline, so rather than dividing the first week back by a near-zero baseline, Runtool rebuilds the windows from the new block. Returning to training does not put you in High Risk on your first run back.
Limitations
- It is a ratio, not a risk measure. The ratio has been criticised on statistical grounds, and the thresholds come largely from team sports rather than from distance running Impellizzeri et al., 2020. It states that the shape of your training changed; it does not predict injury.
- The bands are population findings. They are not calibrated to an individual, a sport, or a training history.
- It says nothing about what the load was made of. Two weeks with the same ratio can be built from very different sessions. Read it alongside the week itself.
- It inherits the load model's limits. Anything that distorts session load, such as heat, illness, or an unmeasured session, distorts the ratio.
FAQ
My ACWR shows Caution, should I stop training?
Caution means recent load is climbing quickly relative to the baseline. It marks a change worth reviewing, not a requirement to stop. A single large session or a missed recovery week often accounts for it, and an easier day or two brings the ratio down.Is a low ratio bad?
Not by itself. A taper produces a low ratio by design, and so does a recovery week. Sustained over months, a ratio below 0.8 means chronic load is falling, which is detraining.Why is my ratio hidden or marked as building?
Either there is not yet 28 days of history, or chronic load is too low for the ratio to be meaningful. Dividing by a very small baseline produces large ratios from small sessions, so those values are not shown.Does non-running training affect it?
Yes. Strength and cross-training carry load, so they count towards both acute and chronic load.Why does my ratio differ from another app's?
Because the inputs differ. The ratio is only as comparable as the load model behind it, and Runtool's load scale is derived from your own heart rate reserve and zones. Runtool also weights both windows exponentially rather than using flat rolling averages.References
- Gabbett, T.J. (2016). The training-injury prevention paradox: should athletes be training smarter and harder? British Journal of Sports Medicine 50(5), 273–280. 10.1136/bjsports-2015-095788
- Hulin, B.T., Gabbett, T.J., Lawson, D.W., Caputi, P. & Sampson, J.A. (2016). The acute:chronic workload ratio predicts injury: high chronic workload may decrease injury risk in elite rugby league players. British Journal of Sports Medicine 50(4), 231–236. 10.1136/bjsports-2015-094817
- Impellizzeri, F.M., Tenan, M.S., Kempton, T., Novak, A. & Coutts, A.J. (2020). Acute:chronic workload ratio: conceptual issues and fundamental pitfalls. International Journal of Sports Physiology and Performance 15(6), 907–913. 10.1123/ijspp.2019-0864
- Williams, S., West, S., Cross, M.J. & Stokes, K.A. (2017). Better way to determine the acute:chronic workload ratio? British Journal of Sports Medicine 51(3), 209–210. 10.1136/bjsports-2016-096589