#foam compression

A few moons ago an awesome Facebook reader asked us about changing shoes and the validity of the "press test".  Here was Eve's link to the press test and here is what it said:

 Do the Press Test

"To determine if the midsoles of your shoes are compressed and are no longer providing cushioning, do the press test. Using your thumb, push on the outsole upward into the midsole. With new shoes, it should be easy to see the midsole compress into lines or wrinkles. As the shoe wears down, the midsole compresses less with the same amount of pressure. When the midsole shows heavy compression lines and the press test reveals a minimal amount of compression, there is little or no cushioning left."

There is a valid point to this test, but it might be considered too rudimentary by many purists.  But most purists rarely can offer us a better solution.  Here is the issue, EVA foam has a lifespan in terms of maintaining its initial shock absorption. EVA foam cells compress and deform over time, most foam in this world does whether it is your Tempurpedic mattress, the foam base for your rugs or your car seat. And with areas of greater wear and compression the foam accelerates its deformation. This is why certain areas of your car seats, your rugs and your bed get softer.  The same thing goes with your shoes. But they really do not get softer, the areas get compressed and the foam changes its density and its integrity. It no longer performs.  Resistance, compressibility and resilience changes.  This is the problem with shoe foam as well, no matter what foam a company is using.  However, the bigger problem if you really think about it is that the foot type you have and the biomechanics (good or bad) that you drive your foam into will be the direction future foot loadings deviate into.  Can this be good ? Rarely. Can this be bad ? Usually.  EVA simply has a given number of cycles, and that number is variable with many factors in place such as weight, running form, foot strike, foot type, weathering of the foam, wet foam, dry foam, outer sole glue, foot bed components and attachment, number of miles.  So, degrading shoe foam is a fact of life for a walker or runner. 

The "press test" gives the user some idea of how much the foam is compressed and how much resilience it has left. But it is a test limited very much by the subjective assessment.  We wouldn't hold a torch to the test and make it a solitary assessment factor, in fact we rarely do it ourselves. But every little test and assessment has some perks and information that can be gleaned from it.

For the record, we like to play it cautious because injuries cost money and time to a runner. So we error on the side of caution always and go for lower numbers for the life span of shoes.  Each shoe is different and we will  not leave you with actual numbers here because the algorithm gets a bit large and convoluted but the bottom line is that cheaper shoes usually use cheaper materials and more expensive shoes usually use better materials (yes, this does not always line up as truth, we know this).  But shoes like Newtons and Altras from our experience seem to survive the trials of running a bit better (at least in our athletes) and so we allow more miles in them.  But for those looking for some harder numbers here are our loose rules:

400-500 miles per pair of shoes.  At 200 miles begin a second pair of shoes and start alternating the shoes every run (old shoe one day, next day use the new shoe).  This will reduce the successive days in a shoe in which the foam is driving deeper and deeper into deformity and thus you are only a day away from a reprieve from the deforming shoe. This will reduce injury risk.  This will also give you a dry shoe on a run the day after a shoe got soaked or caked with mud.  Water and mud add shoe weight and change biomechanics. Once the older shoe his the end lifespan mark you have the second shoe at 200-250 and you are ready for a new shoe. So you are never in a shoe until its death, when it is completely deformed and driving pathomechanics and possible injury only to the very next day step into a new shoe with entirely different (albeit neutral and unbiased) mechanics in the foam.  Injuries occur much of the time with change. Be smarter with your shoe rotation and reduce change.

VITAL NOTE:

Running and walking use different biomechanics and loading styles. Walking has heel strike as a norm, running for the most part shouldn't include heavy or any heel strike depending on the athlete and who's "pulpit of running form" that athlete chooses to pray at the foot of. Like religion, there is no one right way . . .  we each need to find what is best for us. And thus, since running and walking biomechanics are so different you should keep your running shoes for running and have another pair for walking and your other workouts.  Remember though, running shoes are build mostly for sagittal (forward) movement and not for lateral sports. This is why you should never, never ever, use your running shoes for tennis, racquet ball, basketball or many forms of cross fit.  Get a court shoe that is build for lateral movement. Not only will the shoe last longer but it is built on a platform that is more suited for such activity.

Shawn and Ivo