Purpose: Running economy is a key determinant of endurance performance, with recently developed advanced footwear technologies (AFTs) improving running economy substantially. One key feature of AFTs is the thick, compliant midsole. Previously, greater surface compliance has been associated with greater leg stiffness and enhanced running economy, suggesting that increased shoe compliance could induce similar effects and therefore at least partially explain the metabolic benefit of AFTs. However, it remains unclear whether midsole compliance replicates the effects of surface compliance on running economy and leg stiffness and what biomechanical mechanisms underlie these improvements. Methods: Nineteen well-trained male runners completed biomechanical and metabolic testing in two shoes designed to differ only in midsole compliance. Participants ran three 5-min trials at 12 and 16 km·h-1 on an instrumented treadmill in each shoe. During these trials, we collected 3D motion capture data, ground reaction forces, and whole-body metabolic rate via indirect calorimetry. Results: More compliant footwear was associated with a 3.90% improvement in running economy (P < 0.001) and a 2.98% increase in leg stiffness (P < 0.01). Additionally, runners exhibited reduced knee flexion at midstance, leg compression, average knee extension velocity, and peak knee extensor moment with greater midsole compliance (P < 0.05). These biomechanical changes resulted in a 9.46% decrease in average positive knee joint power (P = 0.001). Conclusions: Greater midsole compliance improves running economy. Altering midsole compliance primarily affects knee mechanics and average positive knee joint power. While future research should explore muscle fascicle dynamics to directly determine the muscle-level effect, our results suggest that shoe compliance improves running economy by lowering knee extensor muscle metabolic demand.
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