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The acute:chronic workload model has gained a lot of attention over the past year or so after a series of research papers were published showing it's ability to predict injury in team sports - a full list of these publications can be found here. The model provides a ratio of recent training load (acute) to training over a prolonged period (chronic), usually the most recent week vs the previous four weeks. This can be calculated using a number of different variables - distance covered, weight lifted or minutes*RPE (rating of perceived exertion).
To work out the ratio, the acute workload value is divided by the chronic workload value. So for example if both acute and chronic workload was 3000m the ratio would be 1, whereas if acute was 3000m and chronic workload was 1500m the ratio would be 0.5. The figures below show how this would look when this is calculated in practice, using high speed running distance as the workload measure. Graph A shows acute workload measures over 4 weeks, with graphs B & C showing the addition of the chronic workload after 4 & 5 week respectively. Graph D shows the acute:chronic workload relationship over the course of a full season.
According to the research papers that have been published thus far, an acute:chronic workload ratio of 0.8-1.3 is regarded as the sweet spot, with injury risk being relatively low in this range. A ratio of 1.3 to 1.5 shows a slight increase in injury risk, whilst a ratio of >1.5 is regarded as the danger zone, where injury risk is significantly elevated. You may also note from the figure below, undertraining increases your risk of injury! So having a complete rest week may not be the most beneficial training method, however a tapering week, where workload is lower than usual would be more advantageous from an injury risk perspective.
In terms of applying this workload model to real-life, there are some spreadsheets available online to help with the calculations. It is important to consider cross-training to help maintain training loads whilst injured or when thinking about returning to play.