Classifications Based on Dynamic Navicular Drop during Gait and Characteristics of Flat Foot Muscle Morphology

This study investigates the collapse of the medial longitudinal arch (MLA) as a risk factor for medial tibial stress syndrome (MTSS), hypothesizing that overuse of extrinsic foot muscles to prevent MLA collapse can lead to disability. Twenty healthy adults (39 feet) with a foot posture index score below 6 and no recent lower extremity orthopedic history participated. Ultrasonog-raphy measured foot muscle cross-sectional areas, while three-dimensional motion analysis us-ing VICON assessed foot kinematics during gait, focusing on navicular height at initial contact (ICNH) and dynamic navicular drop (DND) during the stance phase. Hierarchical cluster analy-sis based on ICNH and DND compared muscle cross-sectional areas between clusters using ANOVA or Kruskal–Wallis test. The analysis indicated that ICNH was lower in clusters 1 and 3 than in cluster 2, and DND was smaller in clusters 1 and 2 than in cluster 3. Although there was no significant difference in muscle cross-sectional area between the clusters, the flexor hallucis longus tended to be thicker in cluster 1 than in cluster 3 (p = 0.051). The findings suggest that the flexor hallucis longus may help prevent MLA compression during loading, indicating that over-use of extrinsic foot muscles may contribute to MTSS development.