Thoughts, hopes and reservations of current literature (Part 1)
The Minimalist Index. (Esculier et al., 2015)
So what is it?
A minimalist index is an interesting concept as it came at a period of the re-emergence of minimalist running concepts, the rise in maximalist shoes and shoe options being made available. Not surprisingly, this brought about new experiences and surprisingly, new records and personal bests. With a new frontier to sink their teeth in, scientists in the field sought to understand what these new features mean for the runner. However, there was another problem that needed to be answered. How could we differentiate and categorize all these shoes? (Also a real-life problem for runner in terms of finding a place to store all our shoes)
The minimalist index was hence devised by a group of 42 international experts on footwear and sports* for running shoes. The aim was to develop a way of classifying running shoes and then use it as a standardization for describing shoes in clinical practice and research. The hope was then to use that number and have a general idea on what it meant for the runner.
What is a minimalist shoe?
Although definitions do vary, the more minimalist the shoe, the more the shoe is able to mimic unshod running. Which is running barefoot.
How do you classify it using the index?
A shoe is accessed in 5 categories, where within each category, a score is given. The score is then added together and multiplied by 4 to get a number which serves as a percentage. This percentage then determines how much of the shoe is on the minimalist index spectrum with a higher percentage defining the shoe as more “‘minimalist’’ and a lower percentage defining the shoe as more ‘’maximalist’’.
These categories all aim towards a common denominator which is the influence of these categories on the natural movement of the foot, or unshod, or barefoot.
A lower weight would lead to a higher score.
Running is generally our attempt to progress our body’s mass forward. A heavier shoe would alter your mass, which would ask more of our muscles and accompanying structures to bring it forward. So I guess, it is not really running barefoot if you are wearing additional weights on your legs. David Epstein, a former senior writer at Sports Illustrated and author of the book “The Sports Gene”, described how running was like a pendulum.
Where the more weight you have farther away from your center of gravity, the more difficult it is to swing.
This is the distance of the center of the heel to the bottom of the shoe. For those shoe geeks out there, this entails the shoe liner, the midsole and the outsole of the shoe. A lower overall height would give you a higher score. The concept here is that a higher stack height would cause you to be higher from the ground which is a contrast to, well, no shoes.
With more material underneath your foot, this can affect our proprioceptive control of our limbs when we run, our joint angles and point of impact during the run cycle, This also influences the impact forces and related kinematic variables associated with running. Again, this cannot all be attributed to the height of the midsole but we can see why this was included in the system.
One thing to note is that this only measures the height of the heel and not the height of the midsole at the forefoot. The reason behind this is that with 2 separate measurements and the need to synthesize this value into one criteria, the authors were concerned that this would add to the complexity of the criteria which would lead to differences in agreement rates between the authors on this and its weightage towards the whole scale.
Heel to Toe Drop
The drop is the difference in the height of the heel to the forefoot. Where in this case, the heel height is the measurement from the stack height measurement and the forefoot measurement is stack height measurement at the metatarsal heads of the shoe. (The 1st metatarsal head is commonly described at the location of the ball of the foot).
The difference between the heel and the toe of the shoes can affect the force-length relationship of the surrounding structures of the foot, which can have an influence on other kinematic variables upon gait. It is also possible that a change in heel to toe drop, could affect the center of pressure of initial contact, which would affect other proximal structures.
Motion Control and Stability technologies
This refers to the technologies and proprietary features which are in shoes. Think Asics Trusstic system or the midsole flare of the New Balance Rebel. The index gives examples of such technologies and as long as you recognise these in the shoe, you reduce the score. A lower score would mean more of such technologies in the shoe.
These technologies include:
- Multi-density midsole
- Thermoplastic medial post
- Rigid heel counter
- Elevated medial insole under arch
- Supportive tensioned medial upper
- Medial flare
There has always been a fascination with foot structure and the ideal movement pattern for running. With this, running companies have worked with podiatrists and other sports scientists to produce running shoes that modify movement patterns. Essentially, creating a shoe that acts like an orthosis.
It is interesting to note here that there has been an explosion of new technologies which might not fit into this list of technologies provided by the paper. So when you do use this index, do consider whether the technology that is in the shoe does or intends to modify a runner’s movement pattern.
This refers to whether the shoe is able to bend in multiple directions in relation to the force that is placed onto it. In the index, this is separated into longitudinal and torsional flexibility.
Longitudinal flexibility refers to the ability of the shoe to deform when a parallel force is placed on the back and the front of the shoe. The less it can bend, the higher the score it gets.
Torsional Flexibility refers to the ability of the shoe to deform when a twisting force is placed on the back and the front of the shoe. The more the 2 different parts of the shoe can twist, the higher the score it gets. In reality, you probably would find more flexibility in the forefoot and this is recognized by the index as it attaches the rough degree and orientation the shoe can bend to a score.
The measurement of flexibility comes about with the idea that the shoe is a medium that could restrict the foot’s natural motion in gait. We do know that as we weight absorb during midstance, our midfoot flattens and our toes splay out to shock attenuate. The question here is whether shoes restrict this movement, which is possible. With the addition of the motion control and stability technologies above, the foot’s motion in shod can be altered albeit to a debatable extent.
So how relevant is it to us currently?
A little, maybe. I think it does help to differentiate a certain group of minimalist shoes from the rest, and with this differentiation, maybe we could observe generally (with a very large caveat on the word generally) how these groups of minimalist shoes causes responses in runners in terms of kinematics, kinetics, running-related injuries and performance. But, what is the difference in all those outcomes between a shoe with an index of 20% and 30%. Would you take the index or the individual technologies of the shoe in your clinical decision? It’s hard to really trust the index because there isn’t well demarcated lines within the index.
Another point would be that there are possibly shoes that raise debatable points within the index. Take the Altra Rivera for example. If we score it using the index, it would probably get a high-ish score. Can we take those shoes as a minimalist-esque shoe? But looking at the shoe, at first glance, it doesn’t fit the bill of a classic minimalist shoe. But the kicker here is that the shoe has features and technologies that support the idea of what minimalist running is about. I refer to the use of inner flex grooves and their signature anatomical toe box. Do we then consider it more towards the minimalist range?
The last point I would like to bring up is the distribution of weightage between the components. The fact that we do not know too much into how specific features of the shoes affect us inherently influences how much the index tells us. A 2021 study by Cigoja, Fletcher and Nigg found that altering the midsole bending stiffness on the Nike Free Run 2018 (A minimalist-esque shoe) likely resulted in reductions in metabolic cost by delaying joint work redistribution from distal to proximal joints. Should the flexibility of the shoe have a different weightage? Would it make a shoe more or less minimalist?
A final thought
There are different levels to address when using the index.
Firstly, it is with the index itself. What does the number you get from classifying the shoes mean? If a particular shoe has X%, how can we use that number? What does it tell us?
Secondly, it is with our perception itself with minimalist shoes. What do minimalist shoes mean to clinicians. What are our expectations of the impacts of what minimalist shoes have for the runner and how do they influence our clinical decisions?
Thirdly, the term minimalist-esque shoes should be recognized
Before we can use this index as an efficient tool, we need to have definite answers from all these questions. I do believe this can be a great tool forward in terms of footwear prescription and biomechanics. But if not, it would sadly be stored in the already packed ‘more research required’ pile.
- They did not have a criteria to define an international expert but they are reputable giants!
Cigoja, S., Fletcher, J. and Nigg, B., 2021. Can changes in midsole bending stiffness of shoes affect the onset of joint work redistribution during a prolonged run?. Journal of Sport and Health Science,.
Esculier, J., Dubois, B., Dionne, C., Leblond, J. and Roy, J., 2015. A consensus definition and rating scale for minimalist shoes. Journal of Foot and Ankle Research, 8(1).