A consensus definition and rating scale for minimalist shoes

Background While minimalist running shoes may have an influence on running biomechanics and on the incidence of overuse injuries, the term "minimalist" is currently used without standardisation. The objectives of this study were to reach a consensus on a standard definition of minimalist running shoes, and to develop and validate a rating scale that could be used to determine the degree of minimalism of running shoes, the Minimalist Index (MI). Methods For this modified Delphi study, 42 experts from 11 countries completed four electronic questionnaires on an optimal definition of minimalist shoes and on elements to include within the MI. Once MI was developed following consensus, 85 participants subjectively ranked randomly assigned footwear models from the most to the least minimalist and rated their degree of minimalism using visual analog scales (VAS), before evaluating the same footwear models using MI. A subsample of thirty participants reassessed the same shoes on another occasion. Construct validity and inter- and intra-rater reliability (intraclass correlation coefficients [ICC]; Gwet's AC1) of MI were evaluated. Results The following definition of minimalist shoes was agreed upon by 95 % of participants: "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". Characteristics to be included in MI were weight, flexibility, heel to toe drop, stack height and motion control/stability devices, each subscale carrying equal weighing (20 %) on final score. Total MI score was highly correlated with VAS (r = 0.91). A significant rank effect (p < 0.001) confirmed the MI's discriminative validity. Excellent intra- and inter-rater reliability was found for total MI score (ICC = 0.84-0.99) and for weight, stack height, heel to toe drop and flexibility subscales (AC1 = 0.82-0.99), while good inter-rater reliability was found for technologies (AC1 = 0.73). Conclusion This standardised definition of minimalist shoes developed by an international panel of experts will improve future research on minimalist shoes and clinical recommendations. MI's adequate validity and reliability will allow distinguishing running shoes based on their degree of minimalism, and may help to decrease injuries related to footwear transition. Electronic supplementary material The online version of this article (doi:10.1186/s13047-015-0094-5) contains supplementary material, which is available to authorized users.


A) WEIGHT
Place the shoe on the scale. According to the scale, what is the weight of the shoe (in grams)? 5/5 = less than 125g 4/5 = from 125g to less than 175g 3/5 = from 175g to less than 225g 2/5 = from 225g to less than 275g 1/5 = from 275g to less than 325g 0/5 = 325g and more

B) STACK HEIGHT
Using a digital caliper, measure the height of the shoe at the heel (including insole, midsole and outsole). The caliper must be placed at the middle of the heel when looking at the shoe from the rear end, and in the middle of the heel when looking at the shoe from the side.
Caution must be taken when placing the caliper on the outsole, as the most external (thickest) point of the shoe has to be considered. 5/5 = less than 8 mm 4/5 = from 8 mm to less than 14 mm 3/5 = from 14 mm to less than 20 mm 2/5 = from 20 mm to less than 26 mm 1/5 = from 26 mm to less than 32 mm 0/5 = 32 mm and more

C) HEEL TO TOE DROP
Using a digital caliper, measure the height of the shoe at the metatarsal heads (including insole, midsole and outsole). The caliper must be placed at the middle of the shoe when looking at the shoe from the top. Caution must be taken when placing the caliper on the outsole, as the most external (thickest) point of the shoe has to be considered. Now, substract the height at the metatarsal heads from the stack height to obtain the heel to toe drop. 5/5 = less than 1 mm 4/5 = from 1 mm to less than 4 mm 3/5 = from 4 mm to less than 7 mm 2/5 = from 7 mm to less than 10 mm 1/5 = from 10 mm to less than 13 mm 0/5 = 13 mm and more

Longitudinal flexibility
Using a pinch grip with thumb, index and middle fingers from both hands, apply a superiorly-directed force to the anterior and posterior parts of the shoe. See images below to determine appropriate rating.

2.5/2.5
Minimal resistance to longitudinal bending (the shoe can be rolled on itself more than 360 degrees)

Torsional flexibility
Using a pinch grip with thumb, index and middle fingers from both hands, apply a medially-directed torsional force (pronation) to the anterior part of the shoe. See images below to determine appropriate rating.

2.5/2.5
Minimal resistance to torsion (anterior part of the shoe is turned 360 degrees; anterior outsole faces inferiorly after a complete twist while posterior outsole faces inferiorly) 16 Torsional flexibility

2.0/2.5
Slight resistance to torsion (anterior part of the shoe is turned at least 180 degrees but less than 360 degrees; anterior outsole faces at least superiorly while posterior outsole faces inferiorly) 17 Torsional flexibility

1.5/2.5
Moderate resistance to torsion (anterior part of the shoe is turned more than 90 degrees but less than 180 degrees; anterior outsole faces at least laterally while posterior outsole faces inferiorly) 18 Torsional flexibility

1.0/2.5
High resistance to torsion (anterior part of the shoe is turned more than 45 degrees but less than 90 degrees; anterior outsole can't face laterally while posterior outsole faces inferiorly) 19 Torsional flexibility 0.5/2.5 Very high resistance to torsion (torsional deformation is possible, but anterior part of the shoe reaches less than 45 degrees 20 Torsional flexibility 0/2.5 Extreme resistance to torsion (torsional forces don't significantly change the orientation of the anterior part of the shoe relative to the posterior part)