The sole of the shoe:
With this whole barefoot/minimalist/running mechanics thing exploding right now, one fo the more productive outcomes in science is the realization that the body is smarter than we give it credit for. All those old biomechanical models that presented the body as rigid mechanical body don’t quite accurately reflect what’s going on. Instead, the body works in a nicely complex way where stiffness, tension, and muscle activity are adjusted on the fly based on feedback the body receives. So it’s constantly calculating and preparing for what’s going on. So that means adjusting for the ground surface type, the position of the legs and feet throughout, and so on. Essentially, your body has an in built cushioning system.
You might remember those Adidas shoes that had a computer chip in them that attempted to adjust the cushioning every stride? Well, in this case, the body already does that and better than any technology we have currently can do. (Which makes me wonder, if the shoes adjusting cushioning constantly for the ground, and the body is adjusting the cushioning based on the shoe, among other things, that just seems like a bad situation of constant adjustment going on that is fighting against each other.)
A couple recent studies published in Footwear Science help illustrate this point even more so. First, a study entitled “Relationships between impact variables from running in 20 different footwear conditions” showed that the traditional mechanical tests used to measure cushioning do not translate over to what actually happens cushioning wise when a person is running in those said shoes. While it might be obvious, this is significant because that’s how they classify the “cushioning” that a shoe has….And it doesn’t actually translate to real world application. So we have a situation where the classification doesn’t match what actually happens.
A second article, “Impact characteristics in shod and barefoot running” took several different custom shoes with wide varying midsoles (from 4mm thickness of cushion to 20mm). The basic conclusion was that in terms of impact forces and loading rates the amount of midsole thickness didn’t matter (for statistical significance). And barefoot running changed the impact characteristics largely through an adjustment in foot strike and placement.
The point of the above is to reinforce the idea that the body is a complex dynamic system.
Which brings me to the point of this:
Recently, I decided to cut up a couple different pairs of shoes just to take a look at the insides. It’s rather interesting what certain shoes have in the midsole and the perceived reasoning behind them. You always hear the fancy names thrown around for the technology, but it’s pretty interesting to see it hands on. So that get’s me to the point. If we assume that the following is true:
-We know that the body adjusts stiffness and muscle “tunes” itself to the surrounding environment. So it adjusts based on the surface the foot will hit.
-We know that shoe structure impacts proprioception which changes our in built “cushioning”.
THEN, it would make sense that since our body adjusts for what it is going to hit that the midsole of the shoe influences our bodies adjustment. Therefore if we look at the midsole of the foot, the fact that it Is not uniform begs the question of how does the body adjust to it?
For instance, if we look at running on a soft/unstable surface versus a pretty stable surface like concrete, then we see differences in muscle tuning and preactivation. So, if we have a midsole that has a variety of “stuff” in it, what is the body adjusting for? I don’t have the answer to this question but it seems interesting and plausible.
For example, if we take a look at the two shoes here. One has a firmer outer lining of the shoe with a very soft midsole in the middle. So, does the body prep for the initial striking of the firmer outsole when you either land on your heel or on the lateral edge of the fore/midfoot. Or do you prep for the super soft middle of the midsole?
Similarly, if we look at one of the other examples here, if you are a heel striker, does the firm crashpad get adjusted to, or the soft white midsole or the air pads underneath the forefoot and heel? It seems like the constant change in cushioning would change how our body accurately adjusts or “tunes” to the surface. It’s akin to the story Biomechanist Benno Nigg tells about the Circus performers who install a flexible shock absorbing floor and the injuries skyrocket. Why? Because the way the floor was made you had sections near the “support” stiffer than the furthest points in between the support, which were very pliable and responsive. So you created a situation where you had a way too soft area and then firmer areas, so the body never knew what to prepare for. The injury rate decreased as soon as the floor was changed by the way.
And finally, we get to another shoe design that puts gel basically along the center of pressure of where a heel striker would travel. That seems like a great idea, but again, perhaps you create a level of unstableness and mixed firmness that might create a bit of confusion.
The point of all of this is does the mixing of hardness in the shoe itself create a slightly unstable situation, like if one were to step partially on sand and partially on hard dirt?
I really don’t have the answers to these questions at all. It just strikes me as interesting and I wanted to ask the question. My gut feeling is that creating a highly non-uniform midsole would create a situation where the body doesn’t know exactly what to adjust for and it creates a situation where the foot functions artificially. Because of the variance in hardness you influence the natural motion of the foot in the shoe itself. So it might sink more in certain places or alter the loading in certain areas based on the variance in stability.
I could be off here, but if research from Irene Davis’ group shows that even socks influence the proprioception of the foot, then a crash pad or gel and air insert at various places in the midsole could certainly have an effect in what our body does to prepare for the ground.
Just something to think about.