“Footwear providing minimal interference with the natural movement of the foot due to its high flexibility, low heel to toe drop, weight and stack height, and the absence of motion control and stability devices.”
To come up with the Minimalist Index for a shoe, you assign 0 to 5 points for each of the following categories:
1. Weight. To get the maximum of 5 points, the shoe has to weigh less than 125 grams; you get 0 points for shoes weighing more than 325 grams.
2. Stack height. Basically the thickness of the sole at the center of the heel. You get max points for less than 8 millimeters and no points for 32 (!) millimeters and up.
3. Heel to toe drop. Max points for less than 1 millimeter, no points for 13 millimeters and up.
4. Motion control and stability technologies. Basically all the bells and whistles added to modern running shoes, like multi-density midoles, thermoplastic medial posts, rigid heel counters, elevated medial insole under the arch, and so on. You start with five points, and lose one for every technology included in the shoe.
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 are awarded for 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.
So how do we answer the original question, i.e. what score indicates a true “minimalist” shoe? There’s no single cut-off score, because all the different sub-elements can combine in different ways. Instead, it’s a continuum that can offer guidance about degrees of minimalism. As the authors write, “it could be reasonably hypothesised that transitioning from shoes rated 10 percent to others rated 30 percent within one month is more likely to be safer than switching to shoes rated 80 percent within the same timeframe.”
The other point to mention is that, as it stands, each of the five sub-elements contributes equally to the score. Is that the way it should be? Given that the goal is to “provide minimal interference with the natural movement of the foot,” is shoe flexibility really as important as, say, stack height or heel-to-toe drop? At the moment, there’s not enough evidence to favour one element over others, but that’s an important avenue for future research. For now, this seems like a good place to start, and it’ll be interesting to see how different shoes that I run in rank on this scale.