When is a shoe a minimalist shoe?

The debate around whether or not we were designed to wear shoes has been around for a long time now and has particularly hotted up since the introduction of minimalist shoes in the early 2000's.

What is defined as a minimalist shoes is quite difficult to work out. Is it a pair of flip-flops (jandals, thongs), which have been around since way before the hipster minimalist shoes came along? Certainly people run in them, if just for running for the bus or even in a marathon (yes, he does complete them in the one pair of shoes) .

As it happens, to decide what features were important in determining a minimalist shoe an expert panel of 42 shoe boffins got together, did some number crunching and came up with a list of key features. Then to make it more useful they created a minimalist index system so shoes could be rated and then their respected result can be reported when it is used in a research paper e.g. this research used a minimalist shoe with an MI of 58 for example. 

To come up with the Minimalist Index for a shoe, you assign 0 to 5 points for each of the following categories:
1. Weight. 5 points for when the shoe weighs less than 125 grams (4.4 oz); 0 points for shoes weighing more than 325 grams (11.5 oz).
2. Stack height. This is the thickness of the sole at the center of the heel. Shoes less than 8mm are given maximum points and shoes over 32mm are given 0.
3. Heel to toe drop. Less than 1 millimeter is considered most minimalist and over 13 millimeters is considered a zero score.
4. Motion control and stability technologies.  You start with five points, and lose one for every technology included in the shoe e.g. heel counter equals 1 point, mid-sole shank 1 point, board lasting 1 point, etc. 
5. Flexibility. Half of the points (i.e. 2.5) are awarded for longitudinal flexibility, which is how much the shoe folds up when you press from the heel and toe. The other half is awarded to torsional flexibility, which is how much you can twist the shoe when you rotate the toe in one direction and the heel in the other.

Add up all these points, and you get a score out of 25; multiply that score by four, and you get the Minimalist Index, ranging from 0 to 100.

The Nike Free minimalist shoe

Some key points:

Shoe size: What is a rather bizarre aspect of this paper when you look in both the detailed MI guide or the rating scale, neither of them advise as to what size shoe you should be applying the test to. AS IT HAPPENS, IF YOU READ THE PAPER YOU WILL SEE THAT THE CONTRIBUTORS WERE GIVEN A SIZE US9 MENS SHOE TO EVALUATE. THIS IS NOT STATED IN THE MI GUIDES OR RATING SCALE.

This is a frequent issue in research papers. Whenever you see measurements in mm and mg or inches and pounds rather than percentages, ratios and angles you should immediately be suspicious. In this case we don't need to be suspicious of the results but it would have been better to have given weights and measures as percentages, angles, ratios.

Let's take for example an average men's and women's shoe.To define average you would normally take the running reference shoe being around size 9 US in both the women's and the men's. Immediately you have a problem because a women's 9 US is not a men's 9 US, they are different sizes. In addition, there isn't a universal perfect sizing system for footwear so one manufacturers sizing system can be different from another. Most of us have had the experience of shoes with the same size fitting quite differently. The different sizing will effect all of the measurements that are awarded a weight or a measure.

So, in my case I take a size 12UK (13US). If I weigh my Nike Free in a size 13US it will be significantly heavier than the size 9. The stack height will be greater, the heel pitch will be higher and even the flexibility will be effected due to a combination of thicker materials and altered lever arms. It is difficult for me to work out where my shoe rates in this scale. If I was female and I contacted Nike for weights and heights in size US9 female for my model of shoe I would still have problems because women's 9 is not the same as men's 9. Trying to work out the MI calculations would still be quite difficult as I would somehow need to convert the mens US9 to an equivalent female size and then convert this to the size that I was wearing or wanting to use in my research. 

Natural function of the foot: One of the hardest aspects to determine in a minimalist shoe is whether or not it allows the "natural" function of the foot. There is an endless list of areas to discuss when it comes to this point. An example might be a pair of "thongs" versus a pair Vibram 5 fingers. The pair of thongs I would guess causes an "unnatural" running style versus the 5 fingers but we have to remember your "natural" running style is constantly altering to the surfaces, speed, fatigue, fitness, gradient of the ground, etc. All of these aspects vary from person to person. Any interface which protects or alters skin contact with the ground will alter your "natural" running style. Stimulating or dare I say "hurting" receptors in your skin in the forefoot will increase toe function and your entire lower limb mechanics, just watch anyone walking back to their car across a gravel car park barefoot. You can see it even effects their face as they carefully place their feet and recoil from the discomfort or sharp stones. Picture the same person walking back to their car with runners on. Which gait was more normal. Which one of those gaits was more efficient and less injurious to the joints.

On the flip side, the person walking in barefoot across gravel regularly will strengthen parts of their body they would never strengthen with running shoes on. Equally they could also damage the structures permanently as well. Roll the dice on what you would prefer. 

So overall I like the intentions of this paper and I commend the authors and all those that participated. I think generally this guide is close enough to give us a rough guide, but the implementation in a research or personal situation could be rather clunky.

Esculier, Jean-Francois, et al. "A consensus definition and rating scale for minimalist shoes." Journal of foot and ankle research 8.1 (2015): 42.