The purpose of this study was to investigate the influence of midsole hardness on both impact forces and rearfoot motion. Seven trained male long-distance runners were assessed with a Kistler force plate and with high-speed video, while running at 4.5 ± 0.1 m·s-1 with soft and hard shoe soles (EVA; soft shore Asker C40; hard shore Asker C65). The results showed smaller initial vertical impact peaks, occurring with a higher loading rate, and a significantly larger and faster initial eversion when subjects ran with hard shoes. Support is given to the concept that a more pronounced initial eversion offers an additional deceleration mechanism (Stacoff, Denoth, Kaelin, and Stuessi, 1988) also increasing the eccentric loading of the inverting muscles. On the other hand, during midstance soft shoe soles were found to produce a larger maximum eversion and pronation, also imposing an increased load on the same muscles. So, a good running shoe should be focused on a balance between reducing impact forces and reducing overpronation.