The importance of the foot-posture index compared to other pronation-related variables

Surrogates for pronation, such as the longitudinal arch angle (LAA), the rear foot angle (γ), the Achilles tendon angle (β) and the Foot Posture Index (FPI-6), are often quantified. To date, it is unknown whether the FPI-6 is an appropriate proxy for the LAA, γ or β, and also, whether any of these variables describes pronation. The four key variables (LAA, γ, β & FPI-6) were collected both statically and dynamically. Correlations and a Principal Component Analysis (PCA) were performed. No significant correlations were found between any variables and the FPI-6. The PCA revealed three major components (LAA, β and γ) in the data set. These findings suggest that the FPI-6 does not quantify the same movement as the other variables, LAA, γ and β, which all are assumed to quantify foot pronation.