Clinicians, podiatrists and researchers have been quantifying foot posture and movement in various speed conditions and populations. Common variables to assess foot posture/movement are the Foot Posture Index (FPI-6), Achilles tendon angle (β), rear foot angle (γ) and longitudinal arch angle (LAA). These variables were frequently used in clinical and biomechanical settings. This study aimed to determine the relationship between the biomechanical variables (β, γ & LAA) in static and dynamic conditions and the clinically used FPI-6 and their redundancy. Forty participants performed bipedal standing, over ground walking and running trials. Manual assessment data (FPI-6), kinematic data and ground reaction forces were collected. Discrete biomechanical variables (β, γ & LAA) were calculated at various time points (e.g. heel strike). A Principal Component Analysis (PCA) was performed to quantify the contribution of each variable to the overall variance in the data set. Spearman correlations were used to assess the relationship between the sub-measures of the FPI-6 and the biomechanical variables. Two major components were found that explained 85.2% of the overall variance, consisting of LAA and β variables, respectively. Only LAA variables showed significant, but moderate correlations (r < -0.6) with the fifth sub-measurement of the FPI-6. The LAA and β describe independent movements, which dominate foot posture/movement in static and dynamic conditions. The FPI-6 sub-measurements did not closely reflect static nor dynamic behavior of the rear and mid foot. The FPI-6 and biomechanical variables may not be used interchangeably for screening or grouping individuals according to their foot posture/movement.
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