Running Economy Benefits of Shoes Incorporating Advanced Footwear Technology Decrease With Increasing Incline and Are Negligible Above Moderate Gradients

Shoes incorporating advanced footwear technology (AFT) improve running economy (RE) during level running by ~3%–4% compared to traditional shoes, but the extent to which benefits are preserved on inclines is unclear. Here we investigated the impact of AFT shoes on the energetics of incline running and examined how much benefits are reduced relative to level running. Twelve competitive male runners ran on a treadmill at four gradients (level, 3%, 6%, and 9%) in the Nike ZoomX Vaporfly Next% shoe (VF, AFT) and the Nike ZoomX Streakfly shoe (SF, control) in randomized order. Oxygen consumption was measured using indirect calorimetry, and shoe material properties determined using indentation and cantilever bending tests. The RE benefit of the VF over the SF decreased exponentially with gradient (p < 0.001; R2 = 0.23) from 4.22% during level running, declining by ~20% per 1% increase in gradient, to 2.42% at 3% incline, 1.05% at 6% incline and 0.52% at 9% incline. The VF stored more energy under compression than the SF but had lower resilience. In bending, the VF was ~4-fold stiffer than the SF (4.99 N/mm vs. 1.17 N/mm) with comparable resilience. The exponential decrease in RE benefit with increasing incline suggests that the performance advantage of the VF comes from the elastic properties of the midsole and carbon plate combined, contributing less as the relative importance of elastic mechanisms decreases at steeper inclines. The VF therefore provides the greatest performance benefit on flat courses, with smaller but still present gains on shallow inclines typical of undulating courses.