Mechanical Testing of Midsole Properties at Different Impact Frequencies

Midsole mechanical properties are typically assessed under single-frequency loading conditions, which may not capture how running shoes respond across the range of impact rates encountered during actual running. This study examined the effects of impact frequency on vertical stiffness, peak displacement, and energy return percentage in six running shoe constructions varying in midsole material (ethylene-vinyl acetate [EVA] vs. polyether block amide [PEBA]) and stack height, with hardness varying within the PEBA constructions. Each construction was loaded to a peak force of 1500 N across five impact frequencies (1.5–2.5 Hz). Linear mixed-effects models were used to evaluate the effects of frequency, material, and midsole properties on each outcome. Stiffness increased significantly with frequency across all constructions, while peak displacement decreased with increasing frequency. Energy return percentage also decreased with frequency, although the magnitude of change was small. PEBA constructions displaced more than EVA under equivalent loading conditions, and harder PEBA constructions displaced less than softer PEBA. These findings indicate that impact frequency meaningfully alters the mechanical behaviour of running shoe midsoles and suggest that single-frequency testing protocols may not fully represent how shoes perform across realistic running speeds.