Read running shoe reviews, and you’ll soon come across the statement that you hit the ground with three to five times your bodyweight with every step. Conventional wisdom says these impact forces are a major contributor to injury. After all, three to five times your bodyweight! How can that not hurt you?
One answer, suggests a new study: Because your bones, muscles, and other tissues are built to absorb those vertical forces. What might be more significant, the study says, are the horizontal forces that occur when your foot hits the ground. The study measured what’s known as “peak braking force”—the maximum amount of horizontal force opposite to the direction of travel. This research, just published in the Scandinavian Journal of Medicine & Science in Sports, found that runners with the highest peak braking force were almost eight times as likely to get injured than runners with the lowest peak braking force.
Fortunately, it’s easy to lower your peak braking force and, theoretically, lower your injury risk.
How braking can break you
One interesting thing about this new study: The finding about peak braking force wasn’t the main result the researchers expected at the outset.
Using as subjects relatively new women runners, researchers tested several variables as the women ran on a treadmill at a self-selected “moderately hard” pace, then after that testing the women did a 15-week half marathon training program overseen by the lead researcher, Chris Napier, a University of British Columbia physiotherapist with a 2:33 marathon PR.
Sixty-five women completed the training program. Napier and his colleagues tracked their injury rate, defined as being running-related, related to overuse—as opposed to, say, twisting an ankle—affecting the legs or lower back, and leading to missing three training days within two weeks.
Napier’s primary hypothesis was that average vertical loading rate, or how hard your foot hits the ground with every step you take on a run, would show the greatest correlation with injury rate. But it didn’t. In fact, average vertical loading rate, as well as other measurements of vertical force, had no relation to injury rate.
Only peak braking force did. And that relationship was significant. When the runners were divided into three groups of lowest, medium, and highest peak braking force, those in the highest group had eight times the injury rate compared to those in the lowest, and even five times the injury rate of those in the middle group.
Why might this be the case? “Bone withstands vertical (compressive) forces better than it does horizontal (shear) forces,” Napier wrote in an email to Runner’s World. “Most joints and soft-tissue structures in the lower extremities are built to withstand these same vertical forces, so perhaps they fare worse against the shear forces from increased braking.”
Are you heavy on the brakes?
You might think that hard heel strikers are good candidates for having a high peak braking force. But in Napier’s study, that wasn’t the case. He found no difference in injury rate among types of foot strike (e.g., heel striking versus midfoot striking). Unfortunately, without visiting a lab, you can’t really tell if you have a high peak braking force. “Since the braking force is about one-tenth of the vertical force, or one-quarter to one-third of bodyweight, it is much more difficult to ‘feel’ this force when you are running,” Napier said.
But that doesn’t mean peak braking force isn’t worth addressing, or that there’s nothing you can do about it.
In a follow-up study that has yet to be published, Napier gave runners real-time information on their braking forces. The only cue the researchers gave the runners was to aim to lower their braking forces. All of the runners were able to reduce those forces from high to low.
This article originally appeared on runnersworld.com