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Maybe it’s not the pounding after all.

Since the 1970s, biomechanics researchers have been searching for the telltale traits that predict which runners will get injured and which won’t. Most of their attention, understandably, has focused on the vertical forces that radiate up through the legs each time your foot hits the ground.

But a new study from researchers at the University of British Columbia explores the question from a new angle, linking horizontal braking forces to injury risk. The findings, which were presented at the American College of Sports Medicine conference in Minneapolis last month and now appear in the Scandinavian Journal of Medicine & Science in Sports, bolster the controversial claim that running form is linked to injury risk, and offer some tentative hints on how to run better.

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In the study, 65 female runners visited the Fortius Institute in Burnaby for a detailed three-dimensional gait analysis, which involved running on a treadmill with 42 reflective markers pasted to their head, trunk, and limbs while being filmed from six angles. This allowed the researchers to calculate the various forces experienced by the body at each stage of the running cycle.

The runners then completed a 15-week half-marathon training program, with injuries monitored by a sport physiotherapist.

The researchers suspected that the best predictor of injury would be the “average vertical loading rate,” which is a measure of how jarringly your foot hits the ground. This is a widely studied hypothesis, although studies have produced mixed results on whether high vertical loading rates really predict injuries. As a secondary hypothesis, they looked at a less heralded variable called “peak braking force,” which is the amount your front foot pushes backward horizontally as you land, slowing you down briefly.

During the training program, 22 of the runners suffered injuries – and surprisingly, braking force turned out to be by far the best predictor. When the runners were split into three equal groups based on their braking force, those in the group with the highest force were eight times more likely to sustain an injury compared with the lowest group, and five times more likely than the middle group. None of the other biomechanical measurements, including vertical loading rate, had any significant links to injury risk.

These results are seemingly unexpected, since the vertical forces during running are about ten times greater than the horizontal forces, says Chris Napier, a physical therapist and UBC doctoral candidate who is the study’s lead author. But our bones and other tissues are designed to withstand vertical forces, leaving them more vulnerable to forces acting in other directions.

The findings raise two key questions: First, how do you know if you have excessive braking force? And second, how do you change it?

Coaches often assume that runners who “overstride” – that is, whose feet land far in front of their bodies – will have the highest braking force, especially if they land on their heels. But that’s not necessarily the case, Napier says. In follow-up studies that haven’t yet been published, he and his colleagues have found that the two best predictors of high braking force are running speed and stride length, regardless of where or how your feet land.

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That means slowing down is a simple option to reduce braking force – although not one that most runners are interested in trying, Napier acknowledges. Luckily, taking shorter, more frequent steps – for example, increasing your cadence from 165 to 170 steps a minute, without reducing speed – will also likely reduce braking force. In addition, they found that runners who tried to run “softly” successfully reduced their braking force.

There are already wearable gadgets, such as the Lumo Run, that measure a version of braking force, and tell runners whether it’s higher or lower than normal. Such insights won’t be a magic bullet that prevents all running injuries – “but it’s another piece of the puzzle,” Napier says, alongside other risk factors like how quickly you increase your training from week to week.

The new findings are a reminder that the long-debated links between running form and injuries are more complex than expected – but they do exist. “In running, just like in any task,” Napier says, “how you do it matters.”

Alex Hutchinson is the author of Endure: Mind, Body, and the Curiously Elastic Limits of Human Performance. Follow him on Twitter @sweatscience.

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