Footwear midsole material can have a direct influence on running performance. However, the exact mechanism of improved performance remains unknown. The purpose of this study was to determine if Achilles tendon energetics could potentially play a role in the performance improvements, by testing if changes in footwear midsole stiffness elicit changes in Achilles tendon stretch. Fourteen runners ran in two footwear conditions while kinematic, kinetic, metabolic and ultrasound data were recorded. There was a moderate positive correlation between the difference in stretch and the difference in performance, which was statistically significant (r(12) = 0.563, p = 0.036). Twelve participants had greater stretch and better performance in the same footwear condition. Based on stretch estimates, the difference between conditions in energy returned from the Achilles tendon was 3.9% of the mechanical energy required per step. Energy return of this magnitude would be relevant and could cause the improved performance observed. These results suggest that increasing energy returned from the Achilles could be a valid mechanism for improving running performance due to changes in footwear. These findings lead the way for future research to further understand internal mechanisms behind improved running performance.
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