As runners, we’re lucky that we don’t have to worry too much about blowing out an ACL or breaking an arm. Most of our injuries are caused by overuse: gradual breakdowns or flare-ups that result from repeating the same motion over and over again.
Or are they?
A letter just published in the British Journal of Sports Medicine, from a pair of physiotherapists at the Australian Institute of Sport and the University of Canberra, makes the case that we should dump the term “overuse injury” and refer instead to “training load errors.” They draw on a growing body of research suggesting that how much you train, on its own, isn’t what raises injury risk. Instead, it’s how training load changes that determines your risk.
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The key parameter they focus on is the “acute:chronic load ratio”, which compares your total workload in the most recent week of training to the rolling average of the last four weeks of training.
As it happens, another new paper in BJSM illustrates this concept nicely. The study followed players from an elite rugby league team in Australia (St. George Illawarra Dragons) for two seasons, looking at factors that might predict injury risk.
To calculate “training load,” they used GPS to measure how far the players ran during games and training sessions. The first surprise was that more training didn’t necessarily correspond to greater injury risk when the players had back-to-back matches with less than a week of recovery. In fact, players with a high chronic (four-week average) workload of 18.9 to 22.0 kilometers (close to 12 miles) had a lower injury risk compared to those who trained less.
When they looked at acute:chronic load ratio, players with a ratio above 1.6 (i.e., who had increased their training load by 60 percent compared to their weekly average over the previous four weeks) were far more likely to get injured, by a factor of 3.4 to 5.8. Those who had an acute:chronic ratio above 1.2 also had an elevated risk, though not quite as severe.
The details of what ratio signals danger depends on what exactly you’re measuring. The AIS letter offers a case study using “internal training load,” which is measured by multiplying the length of the training session (in minutes) by a subjective rating of how hard the training session was. They suggest that a ratio above 1.5 is a risk factor for injury.
In some ways, of course, this sounds like common sense. If you’ve been running 50 miles a week consistently, suddenly jumping to 75 miles in one week (an acute:chronic ratio of 1.5) is clearly a gamble. What’s more important is the shift in overall perspective.
Perhaps the best illustration is someone who is used to running 50 miles a week, but for whatever reason—injury, illness, vacation, etc.—has missed some time or been restricted to lower mileage for a month or two. Once you’re healthy (or back from vacation or whatever), it might be tempting to ramp right back to 40 miles—after all, that’s not “high” mileage to you.
But that leap does give you a high acute:chronic ratio. If you get injured doing it, in other words, that’s a training load error, not an overuse problem. It’s a subtle difference, but maybe thinking of it that way will help you avoid making the mistake in the first place.