Plantar fascia plays an important role in human foot biomechanics; however, the morphology and mechanical properties of plantar fascia in patients with flexible flatfoot are unknown. In this study, 15 flexible flatfeet were studied, each plantar fascia was divided into 12 positions, and the morphologies and mechanical properties in the 12 positions were measured in vivo with B-mode ultrasound and shear wave elastography (SWE). Peak pressures under the first to fifth metatarsal heads (MH) were measured with FreeStep. Statistical analysis included 95% confidence interval, intragroup correlation coefficient (ICC_1,1), one-way analysis of variance (one-way ANOVA), and least significant difference. The results showed that thickness and Young’s modulus of plantar fascia were the largest at the proximal fascia (PF) and decreased gradually from the proximal end to the distal end. Among the five distal branches (DB) of the fascia, the thickness and Young’s modulus of the second and third DB were larger. The peak pressures were also higher under the second and third MH. This study found a gradient distribution in that the thickness and Young’s modulus gradient decreased from the proximal end to the distal end of plantar fascia in the longitudinal arch of flexible flatfeet. In the transverse arch, the thickness and Young’s modulus under the second and third DB were larger than those under the other three DB in flexible flatfoot, and the peak pressures under the second and third MH were also larger than those under the other three MH in patients with flexible flatfoot. These findings deepen our understanding of the changes of biomechanical properties and may be meaningful for the study of pathological mechanisms and therapy for flexible flatfoot.

足底筋膜在人体足部生物力学中起着重要作用；然而，柔性扁平足患者足底筋膜的形态和力学特性尚不清楚。本研究对15个柔性扁平足进行了研究，每个足底筋膜分为12个位置，并测量了12个位置的形态和力学性能体内B 型超声和剪切波弹性成像 (SWE)。使用 FreeStep 测量第一至第五跖骨头 (MH) 下的峰值压力。统计分析包括 95% 置信区间、组内相关系数 (ICC _1,1)、单向方差分析（单向方差分析）和最小显着性差异。结果表明，足底筋膜的厚度和杨氏模量在近端筋膜（PF）处最大，从近端向远端逐渐减小。在筋膜的五个远端分支（DB）中，第二和第三个 DB 的厚度和杨氏模量较大。在第二个和第三个 MH 下，峰值压力也更高。本研究发现柔性扁平足纵弓的厚度和杨氏模量梯度从足底筋膜的近端到远端逐渐减小的梯度分布。在横向足弓中，柔性扁平足中第二和第三DB下的厚度和杨氏模量大于其他三个DB下的厚度和杨氏模量，柔性扁平足患者第2、3个MH下的峰值压力也大于其他3个MH下的峰值压力。这些发现加深了我们对生物力学特性变化的理解，可能对研究柔性扁平足的病理机制和治疗具有重要意义。