Impact of Running Shoe Midsole Properties on Energy Cost of Running: Insights Into Midsole Stiffness and Resilience Properties

PURPOSE: The mechanisms behind improvements in running economy (RE) from shoes with supercritical foams (SCF) compared with a traditional midsole material (ethylene vinyl acetate [EVA]) remain only partially understood. This study aimed to evaluate the effect of midsole compliance and midsole resilience of different foams on RE, and spatiotemporal and perception outcomes. METHODS: To separate the effects of midsole compliance and midsole resilience, we compared 3 shoe conditions: EVA40, EVA35, and SCF35, where the number reflects the Asker C hardness, in 17 female runners in a randomized crossover experimental design. RESULTS: RE was similar between EVA40 (14.05 [1.25] W/kg) and EVA35 (14.02 [1.29] W/kg). RE in SCF35 (13.74 [1.22] W/kg) was better than in EVA40 (2.21%) and EVA35 (1.99%). Contact time was longer and step frequency was higher in SCF35, but these differences were small in magnitude. No significant difference was detected in comfort (P = .127) and perceived performance enhancement (P = .608). CONCLUSION: Using EVA35 resulted in a more compliant shoe, with similar resilience compared to EVA40; however, this did not result in significant differences in RE, spatiotemporal, or perceived variables. SCF35 had a higher resilience than EVA35, as intended, but this was accompanied by increased compliance. It appears that the higher resilience in SCF35 condition was the main factor to improve RE compared with EVA conditions (1.99%-2.21%), as compliance differences between the EVA shoes did not affect RE (0.21%). Future studies should compare midsole materials with the same compression stiffness but different resiliency to further evaluate the importance of each characteristic in RE.