Material moderation of plantar impact stress

Abstract

The ability of sport shoe midsole materials to attenuate impact loads was assessed. The three materials examined were mechanically characterized as a stiff spring (STS), subordinate spring/dominant damper (SS/DD), and dominant spring/subordinate damper (DS/SD). Each material was used as the midsole in one pair of basketball shoes. Nine male intercollegiate basketball players performed five vertical jumps in each condition. Vertical ground reaction force and discrete in-shoe stress data were recorded during the landing phase of the jump. Material differences did not alter vertical impact forces. The mean (standard deviation) DS/SD calcaneal impact stress (523.0 (277.4) kPa) was significantly reduced compared to the SS/DD (787.2 (283.7) kPa) and STS (708.9 (394.2) kPa) conditions. In addition, DS/SD calcaneal loading rates were 48.5 and 62.3% of the SS/DD and STS impact rates, respectively. Stress moderation differences across the forefoot were not detected. In vitro modeling suggested improved DS/SD calcaneal impact moderation was founded upon its minimal viscous properties, which allowed greater surface deflection. Lack of discernible forefoot differences were traced to minimal forefoot midsole thickness (0.64 cm) as compared with the rearfoot (1.59 cm). These data indicate the most successful moderation of impact stresses was achieved by the material displaying the least stiffness at in vivo loading rates, providing sufficient material thickness was maintained to allow deflection without bottoming.