Colloquia

Summary

Traction and slip behavior at the shoe-ground interface have a crucial role in the performance of an elite athlete especially during the toe-off phase when propulsion and stability dictate the running economy. Although footwear design and material have advanced in terms cush ioning and energy return, the mechanisms controlling slip initiation and progression remain insufficiently explored.

The main aim of this research was to investigate the tribological behavior of performance footwear designed for running, focusing on the toe-off phase. The goal was to understand how material properties, surface characteristics, and accurate loading conditions extracted from actual running, influence traction and slip.

A combined analytical and experimental approach has been adopted to study the inter action between the soft midsole and outsole materials with textured ground surfaces under controlled loading and sliding conditions. The analytical theories consisted of Hayes’ elas tic layer indentation model and the Westergaard contact model, which were employed to characterize foam compliance and understand contact area evolution. For slipping, Adams’ partial slip theory was used to model transitions between grip, partial slip, and full sliding under tangential loading. These theories, alongside experimental normal loading for contact evolution and traction and slip measurements for tangential loading, provided a mechanistic basis for identifying the critical factors governing slip behavior in running footwear.

The results obtained show that stiffer PEBA foam material had delayed slip onset, with critical transition (partial slip to complete sliding) forces around 200 N in thinner foams (10 and 12 mm thickness), compared to approximately 50 N in thicker foam (20 and 35 mm thickness) which are more compliant. Surface texture influenced traction significantly, with smoother surfaces showing nearly double the slip resistance compared to rougher surfaces. The coefficient of friction at slip initiation ranged between 0.4 and 0.7, decreasing slightly with increasing foam thickness.

This thesis shows a tribological perspective of the shoe–ground interaction during running and provides insights into how material choices and surface characteristics affect traction and slip.