Education

The Shoe Cushioning Myth

Posted By Marty Hughes, DC

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Comments
June 16 2015
Dave Jackson

Have you seen research on the negative effect of flat surfaces? There are absolutely no flat surfaces in nature. My older feet hurt much less when walking on a trail than on a sidewalk. Uneven surfaces create impact at a different part of the foot on every step. Flat surfaces cause impact at the same point of the foot on every step. I have had to add inserts with bumps and points. You are so right. Soft is bad news but so are flat floors and hard flat surfaces that don’t occur in nature.

June 28 2015
Natural Footgear

Hello, Dave,

Thank you for your post. We could not agree more that there is too much concrete and pavement under our feet! We love to hit the trails as often as we can, too, and we are always happiest when our feet feel free within their gear to navigate the terrain as naturally as possible.

You bring up an interesting point about inserts with contouring and how it might feel better than all of the flat we encounter on man-made floors and other surfaces. We firmly believe that most feet are equipped with the shock absorption they need to navigate the majority of the terrain we encounter. While those surfaces are increasingly more flat than not, given a chance, our feet can adjust and support us. Then we can reward them on the trail!

Wishing you happiest of trails,
Sarah K. Schuetz

July 02 2015
Sean

I would think the greater cushioning would increase the impulse (the exact time it takes for force to transfer from one structure to another) when it first comes into contact with the ground. This would slightly lengthen the value for “t” and it would explain why people “feel less impact” and tend to run harder when they are wearing shoes with lots of cushioning. I don’t know exactly how much cushioning would increase the impulse value, but I think it’s worth mentioning.

July 02 2015
Natural Footgear

Hi, Sean,

Thank you for your comment. Here are some follow-up thoughts for you:

The definition of impulse in physics is change in momentum. Isaac Newton actually originally phrased Newton’s Second Law as force equals the rate of change of momentum with respect to time. The impulse, or change in momentum, in this scenario is really determined by the velocity of the foot going down versus the velocity of the foot coming back up (mass matters here too but you have a pretty constant mass over the course of a run). Using a cushioned shoe does change the time over which that impulse occurs, but that doesn’t actually change the impulse. In the same manner, when you get into a car accident, the airbag doesn’t really change the impulse or change in momentum your head undergoes (that is purely determined by the mass of your head and change in velocity). What a cushioned shoe does, or what an airbag does, is allow the change in momentum to occur over a greater time, and that reduces the force needed to make that change in momentum happen.

At first glance, if that is the only factor you look at, it seems like it would be a good thing; in fact, in the case of a car accident, it is! But imagine this scenario: If, upon the installation of airbags in cars, people decided to drive more recklessly and at much greater speeds, then people could totally undermine the benefit of the airbag by doing that. If they are driving faster, the impulse in a collision could be greater, and then despite the airbag extending the time over which the impulse occurs, you could end up with the same force or an even greater force.

So that seems to be what happens in cushioned running shoes. Imagine you are a barefoot runner to start with, but you are given a pair of very cushioned running shoes. You put them on and notice the force is less, so you start to run with a harder impact. Just like the airbags promoting reckless driving, the shoe can mask the fact that you are experiencing a greater change in vertical momentum with each foot strike, like you are driving faster now. Yet in Lieberman’s studies, barefoot runners experienced less force than shod runners—how is that possible? The key here is that the amount of impulse involved in a foot strike is set by three numbers: Mass, velocity on the way down, and velocity on the way up. You are the one that determines that, not your shoe. However, because cushioning allows the impulse to stretch out over a longer time, you experience less force, and then without that feedback, you start to change how you run so that you end up “recklessly” having a greater impulse in each stride and thus the forces get larger in the end. Sneaky, isn’t it?

There is a whole other facet of forces we haven’t even touched on, because the focus has been on the vertical forces. Another interesting chain of cause and effect with cushioned shoes is what goes on horizontally: Because the cushioning reduces the sensation of force, people that heel strike tend to reach out more in front of their bodies. If you freeze frame a video of a lot of runners, at impact their foot is in front of them, not under them (under is more typical of barefoot or minimally shod runners). Why does this matter? Newton’s Third Law says that if you exert a force on something, it exerts a force of equal magnitude on you, but in the opposite direction. If your leg is angled such that it is out in front of you at impact, it almost guarantees some degree of forward impact force on the ground, which means the ground pushes backward on you! That is the exact opposite of what you want to have happen. Imagine repeatedly tapping the brakes while driving and what that does to the smoothness of your ride, not to mention fuel economy! If you look at what the Pose method or Chi Running instructors teach, it is NOT to land with your foot out in front of you but more under you. It isn’t that the shoes make you reach out, but the cushioning lets you get away (at least in the short term) with reaching out because you don’t feel those forces as much. If you try reaching out in front of you with a heel strike when barefoot or minimally shod, you’ll either change your gait soon or hurt your heel. Again, the sneaky thing the shoe does here is not to make you run poorly but instead reduce the sensations that would have otherwise told you that you were running poorly.

Kind regards,
Marty Hughes, DC

August 26 2016
Adrian

I have a high-arched foot and have been told that cushioned footwear is important for this minority foot shape group. Are the “high archers” the exception to your thesis?

August 26 2016
Natural Footgear

Hi, Adrian,

Thank you for your message. And thank you for your question. In short, high arches are no exception. Our philosophy is that most people are born with perfect feet and that any shoe that fails to mimic the natural shape of the human foot and/or prevents the foot from functioning like a barefoot inside the shoe is detrimental to long-term foot health.

Contrary to popular belief, the height of one’s arch is not the issue. What is essential, however, is the levelness of the foot’s support base and how closely the foot is able to interact with the ground. Please check out this article that helps to shed some more light on natural arch support:

www.naturalfootgear.com/blogs/education/17888744-natural-arch-support

I hope this info helps!

Kind regards,
Laura Trentman

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