LAUSR

A recent editorial in the British Journal of Sports Medicine expressed the opinion that particular shoes, e.g. the Nike VaporFly 4%, might provide a performance advantage to distance runners (Burns & Tam, 2019). The authors proposed that the International Association of Athletics Federations (IAAF) modify its rules to place an upper limit on the midsole thickness of racing flats. The assertion from their recommendation is that if midsole thickness were limited to 31.0mm, as they suggested by example, that it would rein in this ‘unfair assistance or advantage’. There is, however, insufficient scientific evidence to support the authors’ proposed upper limit on midsole thickness, also identified as stack height, in racing flats. Indeed, the limited available evidence comparing running economy (RE) in neutral shoes (24.1mm thickness) with maximal-type shoes (35.0mm thickness) does not support the assertion that thicker is somehow performance enhancing (Mercer, Stone, Young, & Mercer, 2018). Similar comparisons using even thinner (‘minimal’) vs maximal soled shoes, or, minimal vs barefoot running (Cheung & Ngai, 2016) report similarly contradictory, or, inconclusive evidence regarding the effect of sole thickness on running economy. Although these studies were not controlled in a manner that allows us to conclusively settle this question, they certainly do not provide support for a proposed regulation to limit stack height. The Burns and Tam proposal is apparently encouraged by reports that a thick-soled shoe performed better than two shoes with thinner soles. In fact, the shoes used as controls in Hoogkamer et al. (2018) were different from one another and did not represent standard or representative controls. To add to the confusion, the test shoes varied in many ways from each other and from the prototype shoe that out-performed them. Any one of a half-dozen uncontrolled variables could have explained or contributed to the differences in RE observed. While it is tempting to attribute these results to the stiff moderator, the differences in energy return, or, even, the thickness of the sole, it is not possible to draw definitive conclusions based on this limited experiment because of the possible influence of uncontrolled variables. These reservations are supported by the work of Flores, Delattre, Berton, and Rao (2019). The Flores et al. study varied sole stiffness and energy return but controlled most other shoerelated features such as mass, upper material, midsole hardness, and geometry. They were not able to find significant group differences in RE attributable to bending stiffness or energy return. This favours an analysis suggesting that the results of the Hoogkamer et al. (2018) comparative study might be due to another uncontrolled factor, or, a combination of factors. One of those factors might be stack height, but there is no evidence to support that assertion. Indeed, indirect evidence would suggest looking elsewhere. We are a long way from being able to draw any evidence-based conclusions about how midsole thickness, or, stack height, effects marathon performance. We do not even have a standard method for measuring stack height. Measurements can vary between those reported by manufacturers, test bureaus, and retailers. For example, forefoot stack height values of 21.0mm (Nike), 24.8mm (Runner’s World Shoe Lab), and 29.0mm (runningwarehouse.com) can be found on-line for the same Nike VaporFly 4% model.