Impact characteristics in shod and barefoot running

Increased impact characteristics are often cited as a cause of running injuries. One method that has been used to reduce impact characteristics is to increase the thickness of the midsole of running footwear with the intention of attenuating greater shock from the foot-ground collision. A second method that has been suggested is to run barefoot. The purpose of this study was to compare the impact characteristics of running footwear of different midsole thickness to a barefoot condition. Three-dimensional kinematic and kinetic data were collected as participants ran at their preferred running speed and at a fixed speed. Impact characteristics (impact peak, time to impact peak and vertical loading rate) were derived from the vertical ground reaction force component. Ankle and knee joint stiffness during the loading phase of support were derived from the change in moment divided by the change in angle. The impact parameters were statistically analyzed using a two-way, repeated measures ANOVA. There were no significant speed by footwear condition interactions. For impact peak, ankle stiffness and knee stiffness, there was no difference among the shod conditions but there were significant differences between the shod and barefoot conditions. Based on their strike index, participants in this study appeared to alter their footfall pattern from a rearfoot to a midfoot pattern when changing from running shod to barefoot. It may be concluded that the change in the impact characteristics is a result of changing footfall pattern rather than midsole thickness.