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The plantar fascia and intrinsic foot muscles (IFM) modulate foot stiffness. However, it is unclear whether the corresponding ultrasonography findings reflect it. This study aimed to examine the effect of the plantar fascia and IFM morphologies on force attenuation during landing and reactivity when jumping in healthy adults (n=21; age, 21–27 years). Thickness, cross-sectional area (CSA), and hardness of the plantar fascia, abductor hallucis (AbH), and flexor hallucis brevis (FHB) muscles were measured using ultrasonography. Single-leg drop landing and repetitive rebound jumping tests assessed the ground reaction force (GRF) and reactive jump index (RJI), respectively. The CSA of FHB was negatively correlated with maximum vertical GRF (r = −0.472, p=0.031) in the single-leg drop landing test. The CSA of AbH was negatively correlated with contact time (r = −0.478, p = 0.028), and the plantar fascia thickness was positively correlated with jump height (r = 0.615, p = 0.003) and RJI (r = 0.645, p = 0.002) in the repetitive bound jump test. In multivariate regression analysis, only the plantar fascia thickness was associated with RJI (β = 0.152, 95% confidence interval: 7.219-38.743, p = 0.007). The CSA of FHB may contribute to force attenuation during landing. The thickness of the plantar fascia and CSA of AbH may facilitate jumping high with minimal contact time.

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