THE EFFECT OF VARYING MIDSOLE HARDNESS ON IMPACT FORCES AND FOOT MOTION DURING FOOT CONTACT IN RUNNING

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 wi th 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 w ith a higher loading rate, and a significantly larger and faster initi al 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, & Stuessi, 1988) also increasing the eccentric loading of the inverting muscles. On the oth er hand, during midstance soft shoe soles were found to produce a larg er maximum eversion and pronation, also imposing an increased load on the same muscles. So, a good running shoe should be focused on a balan ce between reducing impact forces and reducing overpronation.