An article of footwear includes an upper and a sole assembly secured to the upper. The sole assembly includes a lower ground engaging surface portion and an upper surface portion. The upper surface of the first portion is in contact with the upper. At least part of the lower ground engaging surface portion is a polymer material formed from a blend of polybutadiene polymer and a carboxylated butadiene-acrylonitrile or partially hydrogenated carboxylated butadiene acrylonitrile; wherein the polymer material has a density of 1.00-1.25 g/cm3 and a hardness of at least 55 Asker C.