We all have heard the recommendation to change shoes around 300-500 miles of use, but why? Where does this recommendation come from? When running, forces more than 2.5 times your body weight are applied up the leg and through the musculoskeletal system. Footwear has been designed to assist in absorbing these highmagnitude forces in order to improve performance and reduce injury risk. The cushioning in shoes can absorb shock and reduce how fast the force of landing acts on the body.
One of the most popular materials used in the midsole, the main cushioning structure of athletic footwear, is ethylene vinyl acetate (EVA). This material can be manipulated to adjust stiffness and elasticity of the sole of the shoe, thus adjusting its absorption ability and allowing for maximum protection of the foot. Researchers from Tel Aviv University hypothesized that the wearing-down of the midsole would cause a decrease in thickness, leading to an increase in shock-absorption ability. They then simulated a decrease in thickness of 50% and 90%, which resulted in increased heel stresses during running by 19% and 36% respectively.
Prior studies have simulated specific distances, most notably a near-doubling in pressure of the bottom surface of the foot with a 40% decrease in absorption ability (equivalent to a distance of 250-500 miles). All of these studies attribute the changes to structural damage to the EVA within the midsole of the foot.
Although other materials have different abilities to resist wear and tear, the 300-500 mile mark continues to be the general recommendation. However, keep in mind that depending on training, body mechanics and personal injury risk these recommendations can vary from person to person
Even-Tzur, N., Weisz, E., Hirsch-Falk, Y., Gefen, A., & Even-Tzur, Nurit; Weisz, Ety; Hirsch-Falk, Yifat; Gefen, A. (2006). Role of EVA viscoelastic properties in the protective performance of a sport shoe: Computational studies. Bio-Medical Materials and Engineering, 16(5), 289–299.
Dr Cathlin Fitzgerald, DPT, CSCS and Tyler Denn-Thiele, SPT