title: Role of EVA viscoelastic properties in the protective performance of a sport shoe: computational studies.
abstract: Modern sport shoes are designed to attenuate mechanical stress waves, mainly through deformation of the viscoelastic midsole which is typically made of ethylene vinyl acetate (EVA) foam. Shock absorption is obtained by flow of air through interconnected air cells in the EVA during shoe deformation under body-weight. However, when the shoe is overused and air cells collapse or thickness of the EVA is reduced, shock absorption capacity may be affected, and this may contribute to running injuries. Using lumped system and finite element models, we studied heel pad stresses and strains during heel-strike in running, considering the viscoelastic constitutive behavior of both the heel pad and EVA midsole. In particular, we simulated wear cases of the EVA, manifested in the modeling by reduced foam thickness, increased elastic stiffness, and shorter stress relaxation with respect to new shoe conditions. Simulations showed that heel pad stresses and strains were sensitive to viscous damping of the EVA. Wear of the EVA consistently increased heel pad stresses, and reduced EVA thickness was the most influential factor, e.g., for a 50% reduction in thickness, peak heel pad stress increased by 19%. We conclude that modeling of the heel-shoe interaction should consider the viscoelastic properties of the tissue and shoe components, and the age of the studied shoe.
authors.affiliation: Department of Biomedical Engineering, Tel Aviv University, Tel Aviv 69978, Israel.
authors.forename: Nurit
authors.initials: N
authors.lastname: Even-Tzur
authors.forename: Ety
authors.initials: E
authors.lastname: Weisz
authors.forename: Yifat
authors.initials: Y
authors.lastname: Hirsch-Falk
authors.forename: Amit
authors.initials: A
authors.lastname: Gefen
mesh_terms: D003198:Computer Simulation; D004548:Elasticity; D004867:Equipment Design; D019544:Equipment Failure Analysis; D005528:Foot / Q000503:physiopathology*; D018409:Foot Injuries / Q000503:physiopathology* / Q000517:prevention & control*; D006801:Humans; D008954:Models, Biological; D011145:Polyvinyls / Q000737:chemistry*; D012420:Running / Q000293:injuries; D012775:Shoes; D019876:Sports Equipment; D013314:Stress, Mechanical; D014783:Viscosity
chemical_list: D011145:Polyvinyls; C016438:ethylenevinylacetate copolymer