= 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.
Askew et al. (Fri,) studied this question.