The Foot Posture Index is a tool that has 6 observations associated with it to quantify the posture or alignment of the foot during weightbearing. It built on the limitations of previous measurements (which most tend to be unreliable) and observations (which were restricted to only one body plane). The Foot Posture Index observes the posture of the foot in all three body planes.
The advantage and use of the Foot Posture Index is that it takes into account the three body planes. A ‘flat’ or ‘overpronated’ may have a very low arch but a vertical calcaneus. Or a ‘flat’ or ‘overpronated’ may have a normal arch but have a very everted calcaneus. Or another foot may have a lot of medial midfoot bulging and abduction of the forefoot and may or may not have a lower arch and may or may not have a vertical calcaneus. Different classifications will label these feet as either being normal or label them as ‘flat’ or ‘overpronated’ depending on which observation was made. The usefulness of the Foot Posture Index is that it takes them all into account.
In the transverse place it notes the amount of midfoot bulging and abduction of the forefoot.
In the sagittal plane it notes the height of the arch
In the frontal plane it notes if the calcaneus is inverted, everted or vertical.
The amount of the deviation from what could be considered normal is scores from -2 and -1 if supinated and from +2 and +1 if pronated and 0 if about what would be considered normal. The scores are added up to give an overall Foot Posture Index score.