Longitudinal bending stiffness of marathon shoes alters ground reaction force frequency dynamics in elite runners.

Marathon running requires advanced shoe technology. Longitudinal Bending Stiffness (LBS) enhances propulsion efficiency and alters biomechanics, but its impact on Ground Reaction Force frequency characteristics and muscle tuning mechanisms is underexplored. This study enrolled 15 Chinese male elite heel-striking runners. Three LBS conditions (low-LBS (LLBS) 0.31, Mid-LBS (MLBS) 0.40, High-LBS (HLBS) 0.48 Nm/°) were established by adjusting carbon plate thickness. Runners completed 5 trials at 4.8±5% m/s on a 66m track, with ground reaction force (GRF), kinematics, electromyogram (EMG), and subjective perceptions of the shoes collected. Fast Fourier Transform(FFT) was used to analyze GRF, with other metrics also assessed. HLBS had higher vertical force of vertical Force along Z-axis 99.5% Power Frequency (Fz99.5%) than LLBS (p<0.01, d=2.133) and MLBS (p<0.05, d=1.143), plus greater peak loading rates vs. MLBS (p<0.05, d=1.587), indicating better cushioning in LLBS/MLBS.MLBS outperformed LLBS (p<0.05, d=1.566) and HLBS (p<0.05, d=0.357) in vertical Force along Y-axis 99.5% Power Frequency (Fy99.5%), with shorter push-off times (vs. LLBS: p<0.05, d=2.577; vs. HLBS: p<0.01, d=1.667), reduced gastrocnemius activation (vs. both, p<0.05), and elevated propulsive mid-frequency harmonics vs. LLBS (p<0.05).For stability, MLBS/HLBS had lower vertical Force along X-axis Medium frequency (FxMed) vs. LLBS (p<0.05), indicating more concentrated low-frequency mediolateral forces and steadier lateral control. Subjectively, MLBS scored higher in cushioning, propulsion, and stability. Frequency analysis revealed the medium stiffness (MLBS) condition was the best-performing among the three tested settings, with balanced vertical, anteroposterior, and mediolateral force distribution.Findings show medium stiffness optimizes frequency distribution, reducing neuromuscular load and boosting efficiency, offering a new carbon-plated shoe design paradigm.