Exploring Factors to Explain Interindividual Responses to Running Economy in Advanced Footwear Technology Shoes: A Randomised Crossover Laboratory Trial with an Explanatory Predictor Analysis.

BACKGROUND: Advanced footwear technology (AFT) generally improves running economy (RE) measures on average by 4%, although considerable interindividual variability exists. We aimed to explore this interindividual variability in RE response to AFT compared to a CONTROL shoe and identify potential predictors of this variability. Specifically, differences in participant characteristics (e.g., age, sex, anthropometrics, running experience and speed, prior AFT use, V̇O2 peak, lower-limb neuromuscular performance, passive stiffness, running biomechanics, and perceptual measures) between above-threshold and below-threshold responders (∆RE threshold: 2.5%) and how these measures related to RE responses were explored. METHODS: Sixty-four participants (50% female) attended two laboratory sessions at the University of Waikato Sports Science Laboratory in Cambridge, New Zealand, between 10 October 2024 and 7 February 2025. The first session collected baseline information and participant characteristics related to anthropometric, stiffness, strength, and V̇O2 peak. The second session assessed RE variables at 70% of a maximal aerobic test (i.e., V̇O2 peak) speed in two randomised shoe conditions: Salomon S/Lab Phantasm 2 (AFT) and Salomon Aero Glide 2 (CONTROL). RESULTS: AFT significantly improved RE (4.13 ± 1.6%, range: - 2.62% [detrimental] to 11.01% [beneficial]) based on change in oxygen consumption. Significant differences between above-threshold (n = 53) and below-threshold (n = 11) responders were limited to the former exhibiting greater gastrocnemius medialis (GM) stiffness normalised to leg length (mean difference = 33.4 N·m-2, Hedges' g = 0.86 [0.20, 1.51], p = 0.046). GM stiffness normalised to leg length (B = 0.014 N·m-2, p = 0.015) and navicular drop (B = 1.849 cm, p = 0.028) significantly predicted RE responses in our regression model (R2 = 0.273), with greater stiffness (r = 0.35 [0.11, 0.55], p = 0.005) and navicular drop (r = 0.31 [0.07, 0.52], p = 0.012) associated with more beneficial AFT responses. CONCLUSION: AFT enhanced RE in most runners, with few factors explaining the interindividual variance observed. TRIAL REGISTRATION: Australian New Zealand Trials Registry, ACTRN12624000753550, 18th June 2024, prospectively.