Study: Impact Forces Greater in Nike Free than in Pegasus
Researchers surprised by their findings.
Published
July 26, 2013
One of the main criticisms of conventional running shoes raised over the last few years has been that they encourage "hard" landings that increase the impact forces of running, especially on initial contact with the ground.Minimalist shoes, some advocates say, can help reduce impact forces, in part by encouraging a "softer" foot strike and more of a midfoot landing.
A new study by two well-known researchers suggests that, as is often the case in matters of shoes, form, and injury, things aren't that simple.
Richard Willy, Ph.D., of Ohio University, and Irene Davis, Ph.D., of Harvard Medical School, had 14 men run on a treadmill for 10 minutes at a time, at 8:00/mile pace. The men were all rearfoot strikers who were accustomed only to conventional running shoes.
For one of the 10-minute runs, the men wore the Nike Pegasus, which the researchers classified as a standard running shoe. On another 10-minute run, the men wore the Nike Free 3.0, which the researchers classified as a minimalist running shoe. According to the researchers' measurements, the Pegasus' heel height was 36.3 millimeters, compared to 17.6 millimeters for the Free. On the basis of compression tests, the Pegasus provided 31% more cushioning, the researchers said.
While the men ran, Willy and Davis recorded impact forces, stride length and other biomechanical data. They took measurements at the 1-minute mark of the run, and again at the 10-minute mark, to see if impact forces, joint angles, etc. changed during the short run.
The results were the opposite of what Willy and Davis had expected. As they wrote, when in the Free "runners struck the ground with a more dorsiflexed foot [toes pointing toward the shin, meaning more of a heel strike], in more knee flexion, and with higher vertical impact peak and higher average vertical loading rate."
These differences remained consistent throughout the run. That is, as the men--who, again, weren't used to running in a shoe like the Free--continued to run in the Free, they didn't adjust their form to compensate for being in the Free. Their stride length and stride rate remained comparable to when they ran in the Pegasus, and their impact forces remained greater than in the Pegasus.
"Absolutely," Willy wrote in an email when asked by Runner's World Newswireif his findings surprised him.
"We expected that impacts would initially be higher in the minimalist shoe than the standard shoe in the first minute of running. After 10 minutes of running, we expected the reduced cushioning of the minimalist shoe would cause heel discomfort, resulting in a less inclined foot at foot strike, perhaps even a midfoot strike pattern. We then expected a likewise reduction in impact forces. Obviously, our results were completely counter to what we hypothesized."
Willy said that he and Davis used the Free instead of a more barefoot-style shoe, such as the Vibram Fivefingers, because of the popularity of the Free and its positioning as a transitional shoe for runners looking to switch from conventional trainers to barefoot-style running.
"These shoes are widely marketed to the general running public to closely mimic a barefoot running experience to gain many of the benefits of barefoot running, i.e., reduced impact forces, without the increased risk of foot injury due to road debris," Willy wrote. "Due to the popularity of this shoe type and industry claims, we felt that it would be an interesting study.
"Another major reason why we wanted to study this type of shoe is that clinicians often prescribe this type of shoe to patients who are recovering from an impact-related injury to encourage a reduction in impact forces during running. Our findings suggest that this practice may not be advisable, at least in the short term."
"Another major reason why we wanted to study this type of shoe is that clinicians often prescribe this type of shoe to patients who are recovering from an impact-related injury to encourage a reduction in impact forces during running. Our findings suggest that this practice may not be advisable, at least in the short term."
It's important to keep in mind that the runners in this study were unused to minimalist shoes. It's possible that, over time, they might have altered their form in the Frees to lessen the increased impact forces Willy and Davis measured.
To Willy, that fact provides another important take-away from his research.
"Our subjects were novel minimalist shoe runners and they did not receive any instruction on running mechanics," Willy told Newswire. "In order to see a change in running mechanics, a structured training program focusing on changing a component of gait may likely be needed.
"For instance, increasing step rate during running has been shown to reduce ground reaction forces," Willy wrote. "Therefore, runners who wish to transition to minimalism should consider that a thoughtful and directed gait retraining program may be necessary to mimic barefoot running, regardless of the shoe worn."
