September 2010

Impacts spell injury

In the moment: Sports medicine #13927047

Barefoot running seems a solution

By Jordana Bieze Foster

Despite what you may have heard, researchers are a long way from proving that barefoot runners are less prone to injury than those who run in shoes. But data presented at two conferences in August does suggest that barefoot-like gait patterns may be associated with lower injury rates and higher levels of proprioception.

In a study presented at the annual meeting of the American Society of Biomechanics, researchers from the University of Delaware found that increased impact loading was associated with an elevated risk of sustaining a running injury while peak vertical force was not.

The investigators prospectively analyzed 240 female runners, all rearfoot strikers, who averaged at least 20 miles per week. During two years of follow-up, 139 subjects were injured. Of those, the 70 who sought medical attention were compared to 22 subjects from the uninjured group who had no history of injury prior to the study. The most common injuries were iliotibial band syndrome, anterior knee pain, tibial stress fracture, tibial stress syndrome, and plantar fasciitis.

Further analysis revealed that baseline levels of peak tibial shock, vertical impact peak, and vertical average load rate were all significantly higher in the runners who were subsequently injured than in the uninjured subjects. Peak vertical force, however, did not differ significantly between the groups.

The study was originally designed to look at the risk of stress fractures specifically, but the researchers were surprised to find that impact loading rates appear to have a broader influence on running injuries.

“I think the results are really compelling given that all injuries were included. It suggests that impact loading is a global predictor of injury,” said Irene Davis, PT, PhD, director of the University of Delaware Running Clinic and director of research for the Drayer Physical Therapy Institute, who presented the results at the ASB meeting. “That rate of loading is likely to affect loading of soft tissues as well, not just the bone.”

Although the study only looked at rearfoot strikers, Davis hypothesized that the reduced impacts associated with a forefoot or midfoot strike pattern would lower the risk of injury. In a 1980 study in the Journal of Biomechanics, researchers from Penn State University reported that rearfoot strikers experienced two impact peaks during the gait cycle, while midfoot strikers experienced only one.

In their much-publicized January paper in Nature, Daniel Lieberman, PhD, and colleagues referred to this first impact peak as an “impact transient.”  They found that peak impact force during the impact transient period was three times higher in rearfoot strikers, whether shod or barefoot, than in forefoot strikers from Kenya who had never worn shoes. They also found that rate of loading during the impact transient period was seven times lower in the forefoot strikers than in habitually shod rearfoot strikers while running barefoot (but similar to that seen in the rearfoot strikers while wearing shoes).

“You can run fast either barefoot or in minimal shoes on the hardest surfaces in the world with essentially no collisional impact if you forefoot- or midfoot strike,” said Lieberman, a professor of human evolutionary biology at Harvard University in Cambridge, MA, during a keynote presentation at the emed Scientific Meeting hosted by pressure measurement developer novel. “A more natural running style might reduce the frequency of certain types of injuries by reducing impact transients.”

It should be noted that Lieberman emphasized the word “might” with respect to injury risk. Similarly, Davis acknowledged that simply being a rearfoot striker was not necessarily predictive of injury in the Delaware study.

“There are lots of individuals who heel-strike and do just fine,” she said. “But there are degrees of heel strike. My guess is that the greater their dorsiflexion, the higher those impacts will be. “

In contrast to the Kenyan subjects in the Nature study, the words “habitually barefoot” do not describe most American children. But research from the Hospital for Special Surgery in New York City suggests that childrens’ shoe manufacturers should continue to follow the lead of running shoe manufacturers in designing footwear that better approximates barefoot gait.

“Children’s shoe manufacturers traditionally made stiff shoes. But based on the belief that ‘barefoot is best,’ they’ve begun developing more flexible shoes in the past decade,” said Howard Hillstrom, PhD, director of the Leon Root, MD, Motion Analysis Laboratory at HSS, who presented his group’s findings at the ESM conference. “But there’s really no evidence either way.”

Following up on a pilot study presented at the Combined Sections Meeting of the American Physical Therapy Association (see “Stride-Rite taps torsional flexibility”), the investigators analyzed 26 early walkers while wearing four shoes with differing degrees of torsional and toe-break flexibility. Early walkers were defined as children with zero to five months of experience with independent ambulation.

The researchers found that in general, the stiffest shoe was associated with the lowest levels of plantar pressure, while the most flexible shoe was associated with the highest levels of plantar pressure and was most similar to the barefoot condition.  In the previous study, which also included more experienced walkers, the most flexible shoe condition was associated with the lowest number of stumbles.

The increased plantar pressures associated with the flexible footwear are suggestive of improved proprioception, Hillstrom said.

“In adults, especially in diabetic patients, we would want to minimize plantar loading,” he said. “But in these 25-pound kids their load levels are very low.”

Hillstrom noted, however, that torsional and toe-break flexibility are just two of a number of variables that could be affecting plantar pressure and other biomechanical variables.

“It is possible that other structural characteristics of the shoe are also important to their overall function,” he said.

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