Mechanical ageing performance of minimalist and traditional footwear foams

The aim of this study was to obtain a fundamental understanding of how running shoe midsole foam thickness contributes to footwear degradation using the heel and forefoot regions of traditional (TS) and minimalist (MS) running shoes. We hypothesized that ethylene vinyl acetate (EVA) foam midsole material properties and footwear degradation performance under a biofidelic mechanical ageing protocol would differ as a function of shoe type and thickness. Attenuated totalreflectance Fourier transform infrared spectra indicated that the foam chemical compositions were similar and confirmed that all midsoles were composed of EVA copolymer. Differences in density and cell size were detected between shoes and thicknesses. MS foam was uniformly high density (rMS D 240 kg/m3), while TS foam consisted of two co-molded layers with forefoot density (rTS-FF D 250 kg/m3) greater than heel (rTS-H D 160 kg/m3). Relative density and cell size values were generally proportional and inversely proportional to density, respectively. Degradation from mechanical ageing was greatest in the first 2 km of ageing, with the full ageing (21 km) resulting in an average 54% loss of energy absorption. Regardless of shoe type or foam microstructure, thicker and softer heel foams absorbed 83% more energy but degraded ata 49% faster rate. The fact that the heel degraded more rapidly than forefoot caused the drop to decrease at an equivalent rate for both shoe types. Overall, thickness was a greater predictor of average performance than microstructure variables for the present footwear conditions. However, the apparent drawback of thicker foam was exemplified by heel samples, which underwent a 1.4 mm greater loss of thickness and lost 550 mJ more energy absorption than forefoot samples.