New shoe, old shoe. The rotation, it matters.

/

New shoe, old shoe. The rotation, it matters.

At this very moment i am responding to an email of a very sweet and extremely talented runner in Tasmania, I saw her months ago here in the USA as she travelled through. I find myself sharing a conversation with her at this very moment, one she likely knows, but one we all forget, or get lazy with. It is all about
"reducing one more risk factor, reducing one more sudden biomechanical change that can provoke changes in our loading response". 
This is nothing new for veteran Gait Guys brethren here, but we get 100's of newbies here each week, so it is good to remind all.
* Never underestimate the subtle changes in biomechanics that might come from a shoe change in a high mileage athlete. Sweat the small stuff, sometimes." Foam changes, foam loses its resilience with repeated compression cycles, foam deforms into your particular biomechanical loading habits. And sometimes your habitual loading cycles are subtle, but as the foam gives into them, the small thing mushrooms into a significant thing. IT can become a "tipping point" for your clients biomechanics. Something that was initially nothing, becomes something of significance. Help reduce your client's risk factors so you can stay focused on the things that matter, reduce those inner-mind rumbling thoughts of "i wonder if that is a factor". Take those off the table for all your clients, when possible.

We always want to get one more run in on a pair of shoes that is weak and limping its way into the finish line, on its final death throws. 
"Today's story: Bam, i got one more run in on these babies. 
Tomorrow's story: hey i wonder why i am having a little medial foot-arch-ankle pain today???" #facepalm
(not that this has anything to do with the client below, just slamming home my point)

"Dear _____:
Do you think switching to a newer pair of Zante's had any factor in this ? Did the shoe seems to guide the foot differently than the older pair ? Anything feel different ? Sometimes a fresh shoe today changes mechanics too much compared to the one you were just in yesterday (try in the future to have 2-3 pairs in rotation, switch up every run to a different one. Have one newer one in the rotation, another with 200 miles and one that is almost done. That way you are never burning down one shoe and then jumping in a new one. Always be finishing up on an older shoe and starting in on a new one, with one in the pocket in the middle wear milage.)
Now, onto your injury in question . . . . 
-Dr. Allen

More on EVA foam, impact loading behaviors, and adding shoe inserts.

/

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.

_______

Biomed Mater Eng. 2006;16(5):289-99.

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.

________________

J Biomech. 2004 Sep;37(9):1379-86.

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.