Improving running economy through altered shoe bending stiffness across speeds

Increasing the longitudinal bending stiffness of running shoes decreases energy cost until a low point is reached, suggesting an optimal shoe stiffness. Previous research found optimal stiffness varies among individuals, however, none has determined the causes of variation both between individuals and running speeds. The primary purpose of this study was to understand how optimal shoe stiffness is affected by running speed. A secondary purpose examined the anatomical and biomechanical characteristics associated with optimal stiffness variation. Six shoe stiffness conditions were manufactured with a carbon fibre plate between the midsole and outsole of a standard running shoe. Twenty-one male runners (mass = 67.1 ± 5.0 kg, height = 178.9 cm ± 4.0 cm, age = 26.4 ± 8.4 years) completed testing at 2.98 m/s and 4.47 m/s in all shoe conditions, measuring metabolic cost, and several biomechanical and anatomical variables. Data were separated by foot strike. At the fast speed, average optimal stiffness was 19.29 ± 5.62 N/mm with a metabolic benefit of 3.02 ± 2.62%. Slow speed average optimal stiffness was 17.04 ± 6.09 N/mm with a metabolic benefit of 1.93 ± 1.82%. Only rearfoot strikers demonstrated a significant increase in optimal stiffness (p = .04) across speeds. There were no correlations between any of the measured anatomical or biomechanical variables and optimal stiffness. Optimal stiffness varied between subjects but was not correlated to any of our measured characteristics. Rearfoot striking runners may benefit from a higher stiffness shoe at faster speeds to enable optimal performance.