A few weeks ago we wrote about some thoughts on the maximalist shoe foam trend and how it is possible that more foam could mean alterations in impact loading behaviors that could lead to problems (note we used the word could, and not will). If there are pre-existing proprioceptive deficits in a limb these issues most likely will rise to the surface.
The EVA foam in shoes is primarily used to absorb forces via air flow through interconnected air cells in the EVA during shoe deformation under body-weight. When the shoe has seen a finite number of compressive cycles the air cells collapse and the EVA can compact on itself leaving the shoe with an negatively impacting area of compression to fall into. Shock absorption may be impacted and possibly lead to injury.
The Robbins study we discussed a few weeks ago (link) suggested that the reduction of impact moderating behaviour is
Reduction of impact-moderating behavior is a response to loss of stability induced by soft-soled cushioned shoes: Humans reduce impact-moderating behavior in direct relation to increased instability.This is presumably an attempt to achieve equilibrium by obtaining a stable, rigid support base through compression of sole materials. Humans reduce impact-moderating behavior, thereby amplifying impact, when they are convinced that they are well protected by the footwear they are wearing.
These were important points but we wanted to bring to your awareness of the component of the shoe you may have not thought of to this point, the foam foot bed that comes with the shoe, or ones you might add to the shoe yourself post-purchase. With what we have just taught you in our last blog post and this blog post, we will let you make the connection we are suggesting you be aware of when it come to more foam, changes in foam as the shoes and inserts degrade and impaired impact loading behaviors.
There are just 3 brief study summaries here, take the time to read them and read between the lines now that we have educated you a little better in how to think about them.
Shawn and Ivo
J Appl Biomech. 2007 May;23(2):119-27.
Effects of insoles and additional shock absorption foam on the cushioning properties of sport shoes.
The purpose of this study was to investigate the effects of insoles and additional shock absorption foam on the cushioning properties of various sport shoes with an impact testing method.
The results of this study seemed to show that the insole or additional shock absorption foam could perform its shock absorption effect well for the shoes with limited midsole cushioning.
Further, our findings showed that insoles absorbed more, even up to 24-32% of impact energy under low impact energy.
It seemed to indicate that insoles play a more important role in cushioning properties of sport shoes under a low impact energy condition.
Role of EVA viscoelastic properties in the protective performance of a sport shoe: computational studies.
Using lumped system and finite element models, we studied heel pad stresses and strains during heel-strike in running, considering the viscoelastic constitutive behavior of both the heel pad and EVA midsole. In particular, we simulated wear cases of the EVA, manifested in the modeling by reduced foam thickness, increased elastic stiffness, and shorter stress relaxation with respect to new shoe conditions. Simulations showed that heel pad stresses and strains were sensitive to viscous damping of the EVA. Wear of the EVA consistently increased heel pad stresses, and reduced EVA thickness was the most influential factor, e.g., for a 50% reduction in thickness, peak heel pad stress increased by 19%. We conclude that modeling of the heel-shoe interaction should consider the viscoelastic properties of the tissue and shoe components, and the age of the studied shoe.
Heel-shoe interactions and the durability of EVA foam running-shoe midsoles.
A finite element analysis (FEA) was made of the stress distribution in the heelpad and a running shoe midsole, using heelpad properties deduced from published force-deflection data, and measured foam properties. The heelpad has a lower initial shear modulus than the foam (100 vs. 1050 kPa), but a higher bulk modulus. The heelpad is more non-linear, with a higher Ogden strain energy function exponent than the foam (30 vs. 4). Measurements of plantar pressure distribution in running shoes confirmed the FEA. The peak plantar pressure increased on average by 100% after 500 km run. Scanning electron microscopy shows that structural damage (wrinkling of faces and some holes) occurred in the foam after 750 km run. Fatigue of the foamreduces heelstrike cushioning, and is a possible cause of running injuries.