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Can we use the "Acute To Chronic Training Ratio Calculator" to prevent or even predict injury?

The “acute to chronic training ratio” formula is a big improvement over the old ten percent rule. Use it to stay injury free.

So you’re a runner and your training for your big event, then you get injured, you must have been running too much, right? Well it might not be that simple

Recently the sports science and sports medicine community have been rethinking this belief. In some cases, hard training may act as a “vaccine” against injuries making your body more resilient—so doing too little can be as risky as doing too much. The trick is in knowing how much is “too much” and how much is “too little”. The acute to chronic training ratio calculator helps us calculate the thresholds of “too much” and “too little” and provide patients with a “safe training zone”

It allows you to focus on the balance between how much you’re running now and how much running you’ve done over the past month. By tracking this “acute to chronic” ratio, you guard against the twin perils of “too much” and “too little”. The acute-to-chronic training ratio compares your mileage for the last week to your average weekly mileage for the last four weeks. If you’ve run 50, 40, 50, and 60 km’s in the past four weeks, your ratio is 60 (last week’s mileage) divided by 50 (average of last four weeks). That’s 1.2.

In recent studies with athletes from various sports, injury risk climbs when this ratio exceeds 1.2, and increases significantly when it exceeds 1.5. This is a more sophisticated version of the “old 10 percent rule”: If you increased your mileage by 10 percent each week for four weeks, you would end up with a “safe” acute-to-chronic ratio of 1.15. But by looking back for four weeks instead of one, the ratio protects you from overdoing it after periods of missed or reduced training, which leave you more vulnerable when you resume your normal routine.

It is also important to consider intensity. How much you run isn’t the only factor that affects injury risk, because races and hard workouts take a greater toll on your body than easy runs do. You can account for this by calculating a training load ratio. After each run or workout, rate the overall intensity of the session on a scale from 1 to 10. Then multiply that rating by the total duration of the run in minutes to get a more comprehensive measure of training load. Total training load can be separated and measured in both acute and chronic phases of training.

For example: a 40-minute run at an effort level of 6 would produce a training load score of 240.

Now you can calculate your acute to-chronic ratio but using weekly totals of training load instead of kilometres.

Alternatively, some GPS watches and heart-rate monitors calculate a training score for each workout based on duration and average heart rate or speed instead of subjective effort. These are also good options for monitoring acute-to chronic ratio, as long as you stick to one measure for consistency.

If we apply a traffic light system to this method of calculating training then it would be as follows: You can think of a ratio of 1-1.2 as a green light, 1.2-1.5 as a yellow light and 1.5 or above as a red light. But every runner is different, so what applies to the mythical “average” runner may not apply to you. This approach will be most valuable if you keep track of your changing ratios over time while making note of injuries—not just major ones, but also minor aches and niggles. Eventually, patterns will emerge that help you understand which ratios your body can tolerate and at which ratio problems arise.

The obvious time to be careful is when you’re elevating your mileage or intensity to new heights; keeping the ratio low will help you do it safely. But be alert for patterns at the low end, too. You might find, for example, that whenever you let your mileage drop below 20 miles for two weeks in a row, your acute to-chronic ratio spikes a few weeks later when you get back to “normal training” —and that you often get injured as a result. You can’t always avoid injuries, but by looking for patterns, you can at least avoid making the same mistake twice.

Below is a basic calculation table:

Calculate your own ratio in the boxes, then by sharing this information with coaches and practitioners you can help to plan when it is safe return to sport, or increase your training load in preparation for an event.

Week Kilometres Duration Intensity
Week 1
Week 2
Week 3
Week 4

Enter your weekly mileage and training load date for the past four weeks to calculate your acute-to-chronic ratio. Week 4 should be last week, Week 3 the week before it, and so on.

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