The minimalist index is a valid and reliable tool for rating running shoes

Abstract

Background:Minimalist running shoes is a popular topic among physical therapists with regards to biomechanics and injury prevention. Recently, an expert consensus created a new rating scale, the Minimalist Index (MI), which aims at scoring the degree of minimalism of a given shoe. The MI considers five subscales judged by the experts to be important to differentiate between levels of minimalism: (1) weight, (2) stack height, (3) heel to toe drop, (4) technologies, and (5) flexibility. Although such rating scale is needed to improve clinical recommendations, its validity and reliability need to be established. Purpose: To evaluate the validity, and the inter-rater and intra-rater reliability of the MI. Methods: Eighty-five researchers (health or sport sciences), physicians or allied health professionals or specialized running shoe retailers with a minimum of one-year experience in running shoe recommendations were recruited. They all took part in a testing session during which the evaluation of the validity and inter-rater reliability of the MI was assessed. Theywere randomly assigned tofive differentmodels of running shoes (for a total of 10different types of running shoes, 17 to 24 evaluators for each group). They first had to rank (0 to 5) their five assigned shoe models from the most minimalist to the least minimalist. Thereafter, they had to evaluate their five shoes models using the MI. Thirty of the recruited participants also tookpart in a second testing session (intra-rater reliability, test-retest interval of 2 to 14 days) during which they had to rate for a second time the same five shoes. Discriminative validity of the MI was evaluated using one-way ANOVA by comparing the mean scores on the MI according to their rank (1 to 5). Interand intra-rater reliability of the total score of the MI was determined for each type of running shoes using intraclass correlation coefficients (ICC), while Gwet’s AC1 indices with quadratic weighting were used to determine the reliability of the MI’s subscales. Results: One-way ANOVA analysis showed significant rank x score interaction (P< 0.001) with significant Bonferroni adjustments between all ranks (P< 0.001), showing that the MI can discriminate the shoe models according to their ranking. Excellent inter-rater reliability was found for total MI score for each shoe model (ICC= 0.99–1.00, P< 0.001) as well as for weight, stack height, heel to toe drop and flexibility subscales (AC1= 0.82–0.99, P< 0.001). Analyses yielded good inter-rater reliability for the technologies subscale (AC1= 0.73, P< 0.001). As for intra-rater, excellent reliability was found for total score (ICC= 0.99, P< 0.001) and for the five subscales (AC1= 0.88–0.99, P< 0.001). Conclusion(s):These results suggest that theMI is a valid and highly reliable tool to evaluate the degree of minimalism of running shoes. For both intra-rater and inter-rater, total score was found to have excellent reliability, with each subscale showing good to excellent reliability. Implications: The MI can be used by physical therapists to recommend adequate footwear to runners and proper transition time between different models in order to minimize injury risk.