Runners can permanently reduce impact forces through biofeedback.
Irene Davis of Harvard University is one of the world’s leading pioneers of gait retraining for runners. Gait retraining consists of systematically encouraging specific changes in the strides of runners that correct characteristics associated with elevated injury risk. One such characteristic is an esoteric variable called peak tibial acceleration, which is basically a measurement of how hard the runner lands on the ground with each step. In a study coauthored with Harrison Crowell of the U.S. Army Research Laboratory and published in Clinical Biomechanics, Davis used a form of biofeedback to successfully encourage runners to reduce their peak tibial acceleration by half.
All of the ten runners included in the study exhibited higher-than-normal peak tibial acceleration initially. In other words, they were stompers. Davis affixed an accelerometer to the lower leg of each runner to measure tibial acceleration as he or she ran on a treadmill. The information collected by the accelerometer was transmitted to a screen positioned in front of the runners, allowing them to see a simple graphical representation of their impact force. They were instructed to change their stride so that this measurement was reduced by half, bringing it down to within the normal range.
All of the runners were able to do this. Importantly, they were not told how to change their strides to reduce impact. They were given the freedom to manage it in whatever way was most comfortable. Over time Davis gradually withdrew the “crutch” of biofeedback until the runners were maintaining their new, lower-impact strides own their own, by feel. Then they were sent out into the world with instructions to continue running in this modified way. After one month they returned to the lab so that Davis could determine whether they had maintained reduced impact. They had.
It is not known why there is such extreme variation in impact forces between individual runners, but there is, and those who, for whatever reason, land especially hard tend to be highly injury prone. Another running biomechanics expert who works with accelerometers, Stephen McGregor at Eastern Michigan University, told me of a case study involving a member of the Eastern Michigan cross country team. This runner was extremely gifted but constantly injured. When McGregor strapped an accelerometer to him, he found out why. The runner’s vertical accelerations (another way of measuring impact force) were off the charts—the highest of any runner McGregor tested. Neither the runner himself nor his coach had had any idea that he landed much harder than all his teammates.
When I learned of this case I got to thinking. “I am highly injury prone. Perhaps I’m a stomper,” I thought. And when I learned about Davis’s work, I got to thinking even more. Perhaps there was something I could do about it. In a phone interview, Davis told me she believed there was a do-it-yourself version of her biofeedback-based method of reducing impact forces that might work just as well: simply listen to the sound of your footstrikes and try to run a lot quieter.
I did this for about three weeks. While I found it easy initially to make my footstrikes quieter, I was also quickly able to associate the softer sound of my shoe making contact with pavement (or a treadmill belt, which works even better) with particular kinesthetic sensations that then became my primary cues that I was successfully maintaining my softer stride. Specifically, I felt that I was squatting into my stride a bit more than usual, which produced a noticeable increase in tension in my lower quadriceps and a more active use of my glutes to propel forward motion. More generally and abstractly, I felt as though I was now scooting versus bounding (not that I felt I bounded before, but in contrast to my modified scooting stride that’s how my old stride now feels when I revert to it).
Circumstances also gave me an immediate way to test the likely effect of this change on my injury risk. At the time I made the change I was dealing with a half-healed Achilles tendon injury that restricted both how far and how fast I could run. If I pushed either my distance or speed limit too far, the affected area would either become painful during the run, feel sore the next day or both. Over that 3-weel stretch I didn’t made any attempt to run faster or farther than I was doing before, but since changing my stride I didn’t felt any pain in my Achilles during running and the level of all-day soreness steadily diminished.
It wasn’t all good news. I’m pretty sure my new stride was less efficient. It felt a little harder to run at my accustomed paces. This was only to be expected. Research by Stephen McGregor and other running biomechanics experts strongly suggests that runners automatically and unconsciously develop the stride that is most efficient for them, and that any conscious attempt to modify it will reduce efficiency, at least in the short term. This is the reason it’s important that, in her gait retraining methodology, Davis uses biofeedback to allow runners to find their own way to modify a certain stride characteristic instead of telling them to make isolated and overt changes such as landing on the balls of their feet instead of the heels. While even the biofeedback method is likely to reduce efficiency in most cases, forcing runners to consciously emulate a certain ideal form would certainly reduce efficiency even more.
Overall, though, I think my quads and glutes just needed a little more time to adapt to working more than they had to with my previous stride and my neuromuscular system needed time to practice and refine it. In due time it will probably be just as efficient as my old stride, and I may even have a chance to become fitter than in the past because I won’t break down as often. And I should say that, while I use the phrases “new stride” and “old stride”, it’s not like the change I’ve made is great enough to really justify them. The change feels drastic on the inside but an outside observer would probably have a hard time distinguishing the two.
As in the case of that Eastern Michigan harrier, the invisibility of the stride characteristics that predispose some runners to injury also makes it impossible to determine prospectively who’s in need of gait retraining. The only sure indication is retrospective: you get injured a lot.
How about you, are you a stomper?
About The Author:
Matt Fitzgerald is the author of Iron War: Dave Scott, Mark Allen & The Greatest Race Ever Run (VeloPress 2011) and a Coach and Training Intelligence Specialist for PEAR Sports. Find out more at mattfizgerald.org.