Your Acute-to-Chronic (A:C) training ratio

This article was originally posted by Ryan at the original Blogs.


By now, I’m sure most of you know my thoughts on the “10% rule”. It’s far too simplistic. That said, there is such a thing as building up too quickly, right?

Of course there is. It’s just not as simple as this week should always be no more than 10% more distance than last week.

Ideally, we listen to our bodies to know if problems are creeping up but what if we had cues to tell us when we may need to pay closer attention or take concerns more seriously?

Enter the Acute-to-Chronic training ratio (what I will abbreviate as the A:C ratio).

What is the A:C ratio?

It’s simply your past week’s running mileage divided by the average of your past 4 weeks. An example might be helpful.

Let’s say your past 4 weeks have looked like the following:

45 miles

50 miles

55 miles

60 miles

Your A:C ratio is then 60 / 52.5 (52.5 is the average of 45, 50, 55, 60). Rounded off, this is 1.14.

Research has found that, on average, injury risk climbs when you hit a ratio of 1.2 and significantly increases around 1.5.

However, that’s for the “average” person. How many of us are perfectly average? If you look at your history, you’ll probably find that there is some fuzzy area where you cross from being relatively solid with no real injury concerns to where you feel like you’re walking a tightrope.

With this knowledge, you could actually adjust to where you know that, for example, at 1.3 you begin feeling not so great and bad things happen when you get over 1.6. So you can watch for those numbers instead of 1.2 and 1.5.

Also, some suggestions are that you take intensity into account. Not all miles are equal. A 5 mile tempo run is more strenuous than 5 miles very easy. This is a little more complicated to keep track of but might make this kind of ratio even more useful as you can track increases in both volume and intensity.

Why is this better than the 10%rule?

To be honest, this is a similar concept to the 10% rule but, depending on how deep you go, it offers more flexibility.

First, in even its most simple sense, you are given two levels to watch: graduated risk and high risk. It’s not as simple as 9.5% = good, 10.5% = bad.

Second, I like the idea that it doesn’t just take a look at the past two weeks. By looking at the past 4 weeks, it gets a more complete picture of your recent training, not just a very limited snapshot. It would be great to look at 2-3 months, of course, but I get how that would complicate things. 4 weeks seems like a decent compromise.

Third, the idea and, in some circles, encouragement to adjust those numbers to your unique needs is outstanding. Track your history, find the numbers that work for you, then adjust.

Finally, if you go the most complex route and add in intensity, you’re measuring not just volume but total training stress. This is an outstanding, though admittedly much more complex, addition to the equation.

In the end, obviously, we aren’t robots. No numerical formula is going to perfectly predict injury risk. You might get injured when your A:C ratio is below 1 (meaning you’re reducing training load) or you might be able to stay healthy when it’s above 2. At 1.3, you might get injured one time but not another. However, if we can track when our risk is increasing, we can be more aware of that and take more precautions.

Addition to the training log

So what does all of this do for us if we’re not easily able to track it? That’s what I started asking myself and I realized I had the ability to offer a solution.

Starting this morning on the Training Log, you can track your A:C training ratio. It’s right there on your sidebar, easy to see and updated in real time as you update your training log.

Currently, it’s the most simplistic solution. If your ratio is below 1.2, it will show up as green:


From 1.2 to 1.5, it will be orange:


1.5 and above, red:


In the future, I plan to build out this functionality as follows:

First, I want this to be an optional sidebar widget, as are some of the others such as the weather widget, controllable through your log settings.

Second, I plan to make those thresholds customizable. They will default to 1.2 and 1.5 but you will be able to change them to the numbers that work best for you.

Finally, my most ambitious thought is to include intensity. This will take more work as the log doesn’t currently track intensity but I’d like to see it included in the long term future.

As far as I know, the Training Log is the first log to include this A:C training ratio. As always, I am aiming to provide an easy to use, convenient, but useful training log. I hope this is an addition that, while not affecting ease of use, will conveniently add a useful tracking measure.

Photo credit: PSY_1987 by Inland Empire Running Club, on Flickr

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