Brooks Men's Ghost 4-M Synthetic Rubber sole Element - Element is an upper technology featuring superior microfibre construction to pro HPR (High Performance Rubber) is an abrasion-resistant rubber outsole compound that has hi PodTech (Podular Technology) - the midsole and outsole components are engineered to set th HydroFlow  the established cushioning system from Brooks.HydroFlow is a system of chamber BIOMOGO - the bio-degradable midsole - As always, Brooks is a step ahead in innovative pro