Heel–shoe interactions and the durability of EVA foam running-shoe midsoles

Abstract 

A finite element analysis (FEA) was made of the stress distribution in the heelpad and a running shoe midsole, using heelpad properties deduced from published force-deflection data, and measured foam properties. The heelpad has a lower initial shear modulus than the foam (100 vs. 1050kPa), but a higher bulk modulus. The heelpad is more non-linear, with a higher Ogden strain energy function exponent than the foam (30 vs. 4). Measurements of plantar pressure distribution in running shoes confirmed the FEA. The peak plantar pressure increased on average by 100% after 500km run. Scanning electron microscopy shows that structural damage (wrinkling of faces and some holes) occurred in the foam after 750km run. Fatigue of the foam reduces heelstrike cushioning, and is a possible cause of running injuries.

Keywords:  FEA, Foam, Heelpad, Running, Plantar pressure distribution