Effects of Midsole Cushioning Stiffness on Achilles Tendon Stretch During Running

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Abstract

Footwear midsole material can have a direct influence on running performance. However, the exact mechanism of improved performance remains unknown. It is speculated that changes to midsole stiffness may influence the energy return from the Achilles tendon, reducing the metabolic cost. The purpose of this study was to determine if changes in footwear midsole stiffness elicit changes in Achilles tendon stretch, and it was hypothesized that the footwear condition with better running economy for an individual will have greater Achilles tendon stretch.

Fourteen runners with personal best 10km times less than 40 minutes completed two testing sessions. Two footwear conditions were evaluated and consisted of a stiff and compliant midsole. Session one determined the moment arm of the Achilles tendon using dynamometer testing. Session two was a treadmill running session where kinetics, kinematics, metabolic and ultrasound data were collected while participants ran at a submaximal speed in each shoe condition. Main outcome variables were differences in Achilles tendon pseudo-stretch and differences in running economy, quantified as the energy cost of running. Correlation analysis was performed to assess the existence of a linear relationship between the variables.

There was a moderate positive correlation between the difference in pseudo-stretch and the difference in running economy, which was statistically significant (r = 0.563, p = 0.036, d = 0.58). Twelve participants had greater pseudo-stretch and better running economy in the same footwear condition and two participants did not have greater pseudo-stretch and better performance in the same footwear condition.

Based on estimates, the difference in energy returned from the Achilles tendon was 3.8 % on average of the mechanical energy required per step. Energy returns of this magnitude would be relevant and could cause the improved running economy observed. These results suggest that the energy returned from the Achilles could be a valid mechanism for improving running economy due to changes in footwear. These findings lead the way for future research to further understand the mechanism behind improved running economy. Understanding how footwear modifications affect internal mechanisms could have large ramifications on potential strategies for assisting and supporting locomotion.

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