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Farjad Pezeshk A, Sadeghi H, Mousavi Z, Bijari M. Effects of Sports Surface Stiffness on Time–Frequency Features of Vertical Ground Reaction Forces During Hopping. J Sport Biomech 2026; 12 (2) :210-222
URL: http://biomechanics.iauh.ac.ir/article-1-475-en.html

Objective The mechanical interaction between the human body and sports surfaces plays a critical role in maintaining functional stability and minimizing mechanical load. This study ergonomically examined sports surface stiffness within the typical indoor range (300–500 kN/m) and its influence on lower-limb mechanical behavior during a cyclic hopping task.
Methods Thirty male athletes with similar training backgrounds performed hopping across four stiffness conditions. Vertical ground reaction force (vGRF) data were collected using a force plate and analyzed via Fast Fourier Transform (FFT). Peak force, median frequency, and the 99.5% frequency components of the vGRF were compared across surfaces using repeated-measures ANOVA with Bonferroni-adjusted post hoc tests (α = 0.05).
Results Median frequency on the force plate was significantly higher than on all other surfaces (p < 0.001; large effect size η² = 0.437). The 99.5% frequency was greatest on the 500 kN/m surface (η² = 0.348), although the 300 kN/m surface did not differ from the force plate (p > 0.05). Conversely, peak vGRF was lowest on the 500 kN/m surface (p < 0.001; very large effect size η² = 0.654).
Conclusion The 300 kN/m surface produced more favorable force-frequency characteristics, whereas the 500 kN/m surface reduced mechanical loading. Overall, a stiffness range between 300 and 500 kN/m may provide an optimal balance between minimizing potentially injurious loading in both time and frequency domains. These findings offer practical guidance for the ergonomic design of sports flooring and training environments.