Running shoes are designed to absorb the impacts with the ground and thus provide cushioning during the activity. However, the loading cycles involved in normal usage can induce a change in the mechanical properties of the footwear. In the present study, the aging of running shoes over mileage and its effect on running biomechanics are evaluated for two versions of adidas Ultra Boost, differing in the midsole density. The shoes were worn by six subjects that ran 60 km in each shoe. Within this time window, the stiffness (ST) and dissipated energy (DE) of the footwear were measured three times (at 10, 20 and 60 km). At each distance mark, kinetic (ground reaction forces), kinematic (ankle and knee joints three-dimensional angles), muscle activity of the gastrocnemius medialis (GM) and soft tissue vibrations data were collected during running indoor at 3.5 m/s. For both shoe models, the mechanical properties remained unchanged over usage and, accordingly, the biomechanics variables considered resulted constant over distance. The midsole material, which is the element providing cushioning, was also characterized using different techniques. In particular, the mechanical response of thermoplastic polyurethane (TPU) samples was assessed under quasi-static uniaxial compression. The results were comparable to that of the whole sole and the differences found between the two densities were related to the microstructure of the specimens, which was investigated with SEM imaging. Usage did not affect the integrity of the foam structure and clear evidences of aging were not present. Differential scanning calorimetry (DSC) and dynamic mechanical analysis (DMA) were performed to evaluate the TPU behavior with temperature and highlight the physical transitions of the polymer. The results of the material characterization are representative of the behavior of the final object and such approach could be used to achieve an easier and more accurate definition of the shoe response over mileage.
Le scarpe da corsa sono progettate per assorbire gli urti con il terreno e ammortizzare l’impatto durante l’attività sportiva. Tuttavia, i cicli di carico che comporta il normale utilizzo, possono indurre una variazione delle proprietà meccaniche della calzatura. Nel presente studio, l’invecchiamento dovuto all’usura e il conseguente effetto sulla biomeccanica della corsa sono valutati per due versioni di adidas Ultra Boost, che si differenziano per la densità della midsole. Le scarpe sono state indossate da sei partecipanti che hanno percorso una distanza di 60 km in ciascun modello. All’interno di questa finestra temporale, la rigidezza e l’energia dissipata dalle calzature sono state misurate tre volte (a 10, 20 e 60 km). A ciascuna distanza, dati relativi alla cinetica (forze di reazione del terreno), cinematica (la posizione angolare delle articolazioni di caviglia e ginocchio), l’attività muscolare del gastrocnemio mediale e le vibrazioni dei corrispondenti tessuti molli sono stati raccolti durante l’esecuzione di uno sprint a 3.5 m/s. Per entrambe le versioni della scarpa, le proprietà meccaniche sono rimaste invariate con l’utilizzo e, di conseguenza, le variabili biomeccaniche considerate sono risultate costanti nel tempo. Il materiale della midsole, che è l’elemento che assicura l’ammortizzazione, è stato caratterizzato mediante l’utilizzo di diverse tecniche. In particolare, la risposta meccanica del poliuretano termoplastico (TPU) di cui è composta, è stata valutata in compressione uniassiale quasi statica. I risultati sono paragonabili a quelli ottenuti dai test sull’intera suola e le differenze evidenziate tra le due densità sono correlate alla microstruttura dei campioni, analizzata con la microscopia SEM. L’utilizzo non ha intaccato l’integrità della struttura della schiuma e chiare evidenze di invecchiamento non sono presenti. Le tecniche di calorimetria differenziale a scansione (DSC) analisi meccanica dinamica sono state utilizzate per valutare il comportamento del TPU con la temperatura e individuare le principali transizioni fisiche del polimero. I risultati della caratterizzazione del materiale sono rappresentativi del comportamento del manufatto finale e tale approccio può essere utilizzato per ottenere una definizione più facile e accurata della risposta del materiale a seguito del suo utilizzo.
Mechanical aging of running shoes : midsole material characterization and lower limb adaptations
FRIGENI, PIETRO
2016/2017
Abstract
Running shoes are designed to absorb the impacts with the ground and thus provide cushioning during the activity. However, the loading cycles involved in normal usage can induce a change in the mechanical properties of the footwear. In the present study, the aging of running shoes over mileage and its effect on running biomechanics are evaluated for two versions of adidas Ultra Boost, differing in the midsole density. The shoes were worn by six subjects that ran 60 km in each shoe. Within this time window, the stiffness (ST) and dissipated energy (DE) of the footwear were measured three times (at 10, 20 and 60 km). At each distance mark, kinetic (ground reaction forces), kinematic (ankle and knee joints three-dimensional angles), muscle activity of the gastrocnemius medialis (GM) and soft tissue vibrations data were collected during running indoor at 3.5 m/s. For both shoe models, the mechanical properties remained unchanged over usage and, accordingly, the biomechanics variables considered resulted constant over distance. The midsole material, which is the element providing cushioning, was also characterized using different techniques. In particular, the mechanical response of thermoplastic polyurethane (TPU) samples was assessed under quasi-static uniaxial compression. The results were comparable to that of the whole sole and the differences found between the two densities were related to the microstructure of the specimens, which was investigated with SEM imaging. Usage did not affect the integrity of the foam structure and clear evidences of aging were not present. Differential scanning calorimetry (DSC) and dynamic mechanical analysis (DMA) were performed to evaluate the TPU behavior with temperature and highlight the physical transitions of the polymer. The results of the material characterization are representative of the behavior of the final object and such approach could be used to achieve an easier and more accurate definition of the shoe response over mileage.| File | Dimensione | Formato | |
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https://hdl.handle.net/10589/139537