Friday, October 4, 2013

Science Friday: The Ten Percent Rule

As I recover from an injury the questions loom: What caused this? Will I make the same mistake again? Was  "too much, too soon" to blame? I've admitted before that I sometimes break the ten percent rule. Is that where I went wrong?

The ten percent rule--that the safe, injury free way to increase mileage is by ten percent each week--is repeated often by running experts and in training books. It's simple, concise, and easy to remember. But is it true? What does the science say?

Unfortunately for ten percent rule advocates, the science isn't on their side. There aren't many studies that examine the rule, and, to date, none that agree with it. The study that is often mentioned as the best test of the rule was published in 2008 by a group in the Netherlands. 

The participants in the study were 532 novice runners that signed up for a four mile race. The researchers divided the runners into two groups, carefully accounting for past history of sports and previous injuries. The control group was assigned a "frequently used beginners training program" that was 8 weeks long, with an average weekly increase of 23%. (The increases each week varied greatly and included a down week.) The intervention group was assigned a more gradual program that was 13 weeks long, and carefully increased the time spent running by ten percent each week. (One week had only a 2% increase but the rest were pretty consistent.) Both groups were to run three times a week.

The injury rates were shockingly similar. In the control group, 20.3% of runners became injured. In the intervention group, 20.8% were injured. A gentle ten percent increase didn't help.

(Not entirely relevant but interesting: They followed up their work with another study looking specifically at a preconditioning program. Prior research suggested that people who participate in sports that involve jumping and pounding to the joints (soccer, basketball, or volleyball) get injured less often when they start to run than those who participate in sports with less pounding (swimmers and cyclists.) They designed a four week program that simply included a few sessions of hopping mixed with walking. The preconditioning/hopping program was completed before the running program began. The hopping had no effect on running injuries, however, as the control group and the hopping group were injured just as often.)

Recently, a smaller study was published using GPS watches to track increases in mileage. The authors wanted to do away with the subjectivity of subjects self-reporting how far and fast they ran, so they gave them Garmins (Forerunner 110s), asked them to run for ten weeks, and tracked their injury rates. The Garmins couldn't lie: runners who became injured tended to have a greater weekly increase in mileage. But the increases--for healthy and injured--were higher than ten percent; injured runners increased mileage an average of 32% while runners who remained healthy increased mileage by 22%. The difference between the two groups' mileage wasn't quite statistically significant, however, which means that while it's interesting, it's not quite enough to make sweeping generalizations. They also calculated that injured runners had a large (and significant) jump in their mileage the week before symptom onset, with an average increase of 86%*. It may be surprising to the ten percent folks that people can get away with greater than 20 percent increases and remain injury free, but injury following an 86% increase isn't terribly shocking.

The authors of the GPS study summarize nicely: "No clear evidence for safe progression of weekly volume exists." The ten percent rule may be convenient, but it's not accurate. In fact, some people can safely increase weekly mileage over 20 percent and remain healthy. But others (1 in 5 in the first study) get injured even with an increase of just 10 percent. It seems like it's up to you to know your body and what it can handle, which unfortunately is learned by trial and error (and injury.) It's probably best to err on the side of caution, and maybe not try to bump up by 86% in one week. 

Dream big, 

*It should be noted that the number of participants in this study was small, and the number of injured was even smaller, only 13. So while this may seem like a high number, it's probably because of the small number of people. 

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