Flatfoot vs Pronation: Structural and Functional Dissociation Between Foot Posture and Foot Physiology.

BACKGROUND: In clinical practice, static foot posture is frequently interpreted as a proxy for foot structure and function. The Foot Posture Index-6 (FPI-6) is a descriptive tool, often used to infer the presence of flatfoot and abnormal dynamic loading. However, the extent to which static pronation reflects structural arch collapse and functional plantar loading remains unclear.

METHODS: This secondary analysis examined the concordance between static foot posture (FPI-6), structural flatfoot assessed through the Arch Index (AI), and dynamic plantar loading during gait. Data were derived from a previously published cross-sectional data set including 100 healthy adults. Feet were classified as pronated when FPI-6 ≥ +6 and as flatfoot when AI ≥ 0.26. Dynamic plantar pressures were recorded during walking using a pressure platform, with regional peak pressures and vertical forces extracted for the hallux, metatarsals, midfoot, and heel. Associations between measures were explored using Spearman correlations and χ² tests.

RESULTS: Among 200 analyzed feet, 43.2% were classified as pronated by FPI-6, whereas only 19% met flatfoot criteria according to the AI. Overlap between classifications was limited, with only 16.3% of pronated feet also showing structural flatfoot. No statistically significant association could be detected between FPI-6 and AI (ρ = -0.05; P = .445).Distinct dynamic loading patterns emerged, with higher FPI-6 scores associated with increased hallux loading and reduced fifth metatarsal loading, whereas higher AI values correlated with greater central and lateral forefoot loading. These patterns suggest limited concordance between static posture, structural morphology, and dynamic loading characteristics.

CONCLUSION: In this asymptomatic sample, within the present data set, static pronation assessed with the FPI-6 was not significantly associated with structural flatfoot (P = .445) and showed limited concordance with dynamic plantar loading patterns. These findings should be interpreted with caution given the cross-sectional design and the potential nonindependence of bilateral observations. A multidimensional assessment integrating static posture, structural measures, and dynamic analysis may provide a more comprehensive framework for clinical evaluation.

LEVEL OF EVIDENCE: Level III, diagnostic, cross-sectional study.