Biofidelic mechanical ageing of ethylene vinyl acetate running footwear midsole foam

Abstract

Simple manipulation of force–time input is a potential strategy to increase the biofidelity of running footwear mechanical ageing. The purpose of this study was to compare a Dwell protocol, which incorporated a recovery period characteristic of the float phase of running, to traditional sinusoidal ageing. A second aim was to use the protocol to compare the mechanical ageing performance of foam, a halved commercial running shoe, and a cylindrical plug cut from a running shoe to quantify effects due to testing geometry and to estimate the contribution of midsole foam to shoe energy management. Dwell was more biofidelic and less aggressive than Sine because (1) net displacement and energy absorption were greater than Sine and (2) net displacement and energy absorption decreased at a slower rate than Sine. Using a 60 mm diameter cylindrical plug to estimate the performance of a halved shoe with 100 mm contact area led to 20% overestimation of energy absorption. Comparing the performance of a slab of foam and a cylindrical plug cut out from a shoe, the midsole was estimated to manage 90% of the energy. Differences between sample types were also related to stiffness, yield behavior, and resulting hysteresis curve shapes, which revealed that the outsole improved sample deformation and durability. Overall, results supported that Dwell improved the biofidelity of mechanical ageing, testing geometry is an important consideration in experimental design, and most of the energy was managed by the midsole.