Background Distance running has been revolutionised by the introduction of modern running shoes, named advanced footwear technology (AFT). AFT shoes commonly implement carbon fibre plates to increase longitudinal bending stiffness as well as highly responsive midsole foams, which increase energy return. In 2022, World Athletics stipulated that midsoles in competition road running shoes cannot exceed 40 mm in thickness, but this limit seems arbitrary. Therefore, this study aimed to understand the effect of midsole thicknesses in AFT shoes at (40 mm) and above (50 mm) the current regulation on running economy (RE), perceived comfort and exertion, and spatiotemporal variables during treadmill and overground (400-m track) running, while comparing values to those of an entry level running shoe (EL). Results On the treadmill, RE with the 40 mm shoes improved by 2.4 ± 1.1% compared with the EL shoes (56.0 ± 4.3 ml x kg- 1 x min- 1 vs. 57.4 ± 4.2 ml x kg- 1 x min- 1, P < 0.001) and improved further with the 50 mm shoes (55.7 ± 4.3 ml x kg- 1 x min- 1, P = 0.042, -0.6 ± 1.2% vs. 40 mm). A similar pattern was seen during overground running, but no correlation was detected between the magnitude of differences for each pair of running shoes and running surface (P > 0.36). Both AFT models showed lower perceived exertion compared with the EL shoes, but no differences were detected between the AFT models. The 40 mm shoes were seen as more comfortable than both the 50 mm- and EL shoes on the treadmill as well as on the track. No differences were detected between any of the shoes with respect to ground contact time, step frequency, or step length. Conclusion Increasing midsole thickness beyond current regulations can further improve RE, although it remains to be seen whether such differences would translate into performance. The 40-mm limit for midsole thickness imposed by World Athletics for road running shoes does not represent an upper limit beyond which no further gains can be obtained and therefore could be limiting the role of footwear in performance breakthroughs.
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