500 Miles/800 Km May Not Be Point Of Degradation For Running Shoe Midsole Material Properties

Advances in running shoe technology have focused on midsole foam compliance and resilience-factors that influence running performance outcomes. While conventional guidelines recommend replacing shoes every 500 miles/800 km, the degradation of novel shoe foams with prolonged use remains poorly understood. PURPOSE: This study aims to assess change in shoe midsole material properties in response to wear while accounting for variations in body mass. METHODS: Nine pairs of self-identified worn running shoes (two common models from the same company) were collected from collegiate runners (distance: 850.2 ± 218 km.; body mass:58.0 ± 8.0 kg). New shoes of the same model were used as zero-distance controls. Shoes were subjected to 100 cycles of uniaxial loading in the heel to simulate a common impact transient (557.2 N). Stiffness, energy return, net displacement, and a runner mass-scaled wear score (body mass x mileage) were calculated for each shoe. Pearson’s r correlation coefficients were used to assess the relationships between wear score and shoe material properties (0-0.39 weak, 0.40-0.59 moderate, and 0.60-1.0 strong). RESULTS: There were weak associations between wear score and displacement, stiffness, and energy return for the first model (r-range: .09-.15, p ≥ .05). There were strong correlations between wear scores with displacement (r = .88, p = .004) and stiffness (r = -.89, p = .003), yet a weak association with energy return (r = -.36, p = .38) for the second model (Fig). CONCLUSIONS: We identified variations in material response to wear between shoe models from the same company, suggesting that a 500 mile/800 km replacement guideline may not be universal, and instead may be shoe-dependent.